PS, suggestion to put in your notes if using a "longPress", also need to change the startDrag to a much bigger number (than 4;) or the long press never gets called.

//when using long press also increase startDrag 
marker.OnLongPress += OnMarkerLongPress;
OnlineMapsControlBase.startDragDistance = 40;

But this requires a slight modification of the source code Online Maps.
OnlineMapsTileSetControl.cs (line: 1600) from:

OnlineMapsTile currentTile = tile;

to:

OnlineMapsTile currentTile = tile;
if (currentTile != null && currentTile.status != OnlineMapsTileStatus.loaded) currentTile = null;

OnlineMapsBuffer.cs (line: 421) from:

if (!tile.hasColors)

to:

if (!tile.hasColors || tile.status != OnlineMapsTileStatus.loaded)

I give you changes so that you do not wait for the new version to be available.
The next version of Online Maps will contain these changes.

Script:

using UnityEngine;

public class DetectNoDataTiles:MonoBehaviour
{
    private Color32 c1 = new Color32(204, 204, 204, 255);
    private Color32 c2 = new Color32(201, 201, 201, 255);

    private void Start()
    {
        OnlineMaps.instance.OnChangeZoom += OnChangeZoom;
        OnlineMapsTile.OnTileDownloaded += OnTileLoaded;
        OnlineMapsCache.instance.OnLoadedFromCache += OnTileLoaded;
    }

    public void OnTileLoaded(OnlineMapsTile tile)
    {
        Color32 p1, p2, p3;
        if (OnlineMaps.instance.target == OnlineMapsTarget.tileset)
        {
            p1 = tile.texture.GetPixelBilinear(0, 0);
            p2 = tile.texture.GetPixelBilinear(0.5f, 0.5f);
            p3 = tile.texture.GetPixelBilinear(1, 1);
        }
        else
        {
            p1 = tile.colors[0];
            p2 = tile.colors[32896];
            p3 = tile.colors[65535];
        }
        

        if (ColorEqual(p1, c1) &&
            ColorEqual(p2, c2) &&
            ColorEqual(p3, c1)) tile.status = OnlineMapsTileStatus.error;
    }

    public static bool ColorEqual(Color32 c1, Color32 c2)
    {
        if (c1.r != c2.r) return false;
        if (c1.g != c2.g) return false;
        if (c1.b != c2.b) return false;
        if (c1.a != c2.a) return false;
        return true;
    }

    public void OnChangeZoom()
    {
        Debug.Log("zoom" + OnlineMaps.instance.zoom);
    }
}

As a side note, I don't seem to be getting zoom in completed notification either, related ?

void Start () {
        //few other things
        OnlineMapsLocationService locationService = OnlineMapsLocationService.instance; //used in other code
        OnlineMaps.instance.OnChangeZoom += OnChangeZoom;
        OnlineMapsTile.OnTileDownloaded += OnTileDownloaded;
        OnlineMapsCache.instance.OnLoadedFromCache += OnLoadedFromCache;
//more stuff
}
    public void OnTileDownloaded(OnlineMapsTile tile)
    {
        Debug.Log("Tile DOwnloaded at zoom: " + OnlineMaps.instance.zoom);

        Color colTextPix;
        colTextPix = tile.texture.GetPixel(1, 1);
        Debug.Log("colour of pixel downloaded" + colTextPix);
    }

    public void OnLoadedFromCache(OnlineMapsTile tile)
    {
        Debug.Log("Tile from Cache zoom: " + OnlineMaps.instance.zoom);
        Color colTextPix;
        colTextPix = tile.texture.GetPixel(1, 1);
        Debug.Log("colour of pixel cached" + colTextPix);
    }
   
        public void OnChangeZoom()
    {
        // When the zoom changes you will see in the console new zoom.
         Debug.Log("zoom" + OnlineMaps.instance.zoom);
        //debug never reports "zoom" change above
       //code below not run
        if (OnlineMaps.instance.zoom > HZoomMax) OnlineMaps.instance.zoom = HZoomMax;
        HZoomCurrent = OnlineMaps.instance.zoom;
        testZoomedImage();
    }

Output: note: zoom 20 has no output, but shows "no data" on screen and zoom 20 in "map" inside inspector:
Debug (shortened version, as get returns from several tiles each time moved or zoomed):

Tile from Cache zoom: 18
colour of pixel cachedRGBA(0.784, 0.753, 0.702, 1.000)
Tile from Cache zoom: 19
colour of pixel cachedRGBA(0.831, 0.820, 0.753, 1.000)

any suggestions greatly appreciated ?
PS will try again with new version from today

Yes, it is possible to implement, for example, in such ways (below).
The general idea is that all "no map data" tiles have the same image.

1. Intercept the request to download the tile, download the tile and check the checksum of the data. If the checksum matches the "no map data" tile, then set the status - error.
http://infinity-code.com/doxygen/online … 5a04ae9f94
2. Subscribe to OnlineMapsTile.OnTileDownloaded and OnlineMapsCache.OnLoadedFromCache. When the tile is loaded, check the colors of several pixels (for example, 3-5 pixels in different places of the image). If the colors of the image match the colors of "no map data" image, set the status - error.

In a separate post, @Alex noted that providers differ in the data zoom levels supported.

eg,
Currently:
user zooms in ... gets "no map data page"
User has to zoom out again to get map back.

Max zoom can be restricted but this is not dependent on the specific map location ... map areas (in the world) have different max zoom levels. Some are 15, some 18 etc.

Proposal:
user zooms in ... gets "no map data" page
Code detects "no map data" and automatically zooms out slightly.
code then restricts max zoom (existing function), so the "no map data" doesn't happen again while operating in this location.
User zooms in ... code puts map at max zoom available with map data.

PS, forgot to mention, fantastic product and great support with updates, you have solved many of my issues over the last 9 months via reading docs, examples and others forum posts.