Just tried to edit an armor mesh as a test, but it appears invisible in-game. Would it work like this?
1) Convert the .gr2 to collada (.dae) via GR2ConverterDAE, with the "Divinity: Original Sin EE" profile. Default "Export Options" and "GR2 Export Options" were used.
2) Import the .dae in 3DX Max 8 and do the edit (I just deleted some vertices).
3) Export to .gr2. I tried the default options, and also the options explained in the step 6 of the "Importing and Exporting GR2 Models" tutorial.
4) Optimize mesh with GR2Preprocessor.
Using the .smd format, as explained in the GR2 Models tutorial, made the .gr2 exporter plugin crash, I didn't use the Granny Viewer (as in the step 4), though, as I thought it wouldn't be necessary, since it isn't in this tool package.
Some minor pointers would be enough for me to fiddle around a bit more