There was a french user (MGCaladtogel) who created a script so the game would be isometric a while back.
Topic can be viewed here although it is in french:
http://rpgmakerxp-factory.net/forum/ind ... ic=10787.0
Gudib is now implementing/translating that script, well has already. Into his TBS (GTBS) and now with the help of Siegfried (another french RM user on that board who made a demo- I can't find the link right now, which implemented height) is implementing height into his TBS aswell.
Both Siegfried and Gudib have accounts on this site.
As for the script itself, it works by having 2 maps and putting the image of the first map over the second and changing the angle of the second by 45 degrees.. I think (it's in french and I'm not a scripter so this is what I assume)
The first map is the foreground image which is made up of whatever isometric tileset you make, and drawn on like any other tileset however is only a visual layer. Then you have the second map which is a standard rtp or any tileset where you create the outlines of the isometric to where the boundaries are. It takes a bit of trial and error figuring out where you need to put the boundaries though.
The isometric tilesets take a bit of work getting them in right positions, however most of my template is complete and now the hardest part is finding Ideas to fill in the gaps and map the maps look less dull. And of course touching up.