S3D makes much of it quite easy.
In terms of the land-units, you want to make it a fixed, bombed out area, is that it?
http://sh4.skwas.net/docs.aspx
You'll want to clone a ground unit, probably.
http://sh4.skwas.net/manual/remap
LEt me know what you're getting at, and I'll show you where to start. Make a thread about it, then walk others through so they can see it and perhaps get new ideas.
One cool thing about S3D is not just doing the modding, but seeing things frequently gives you new ideas