oops, sorry about that, need to get my eyes tested...
Interesting idea but I don't think there is any way to embed a web based gizmo like that in the minetest client,
certainly not via the modding api at any rate.
What you could do, however, is divide a large map up into tiles and configure the co-ordinates of each one.
Possibly a customized minetest mapper could do this automatically and even produce the necessary config file.
The only problem with having a lot of maps would be the time required to download them all to the client,
although it may not be such a big problem for the server I you are using CURL.
Edit: I've done a few quick calculations and based on the example maps I made at 320x320
the mapper generated map tiles average at around 80Kb after png crushing
So for a 6400 square map that would be 400 tiles weighing in at approx 30Mb
That is not altogether out of the way but would be a PITA to download for the first time.
You could possibly use a lossy compression algo to shrink the filesize even more and
make higher quality images available as a texture pack that users can download themselves.
Problem there is keeping things up to date as maps grow and evolve, just some ideas.