I'm not familiar with warhammer specifically, but I like the idea. My first image of this game while you were laying it out was:
The actual sketchup file would only need to be e-mailed twice, once from each person. After that, it would be like an old Chess-By-Mail game, like you said the spaces would have to be numbered/lettered. In this model you could even play through instant messenger or IRC chat or something.
--First player sets up an 'Environment' on a pre-made, universal Numbered+Lettered game grid: Basically they just throw the scene together, maybe with trees and boulders and mountains and stuff, these environmental obstacles would affect movement rates for all or certain units.
--First player then adds their "Army" to their end of the playing board. Then sends the sketchup file to Player Two.
--Player Two sets up their "Army" on their end of the playing board and sends this file back to Player One.
___At this point, there is no need to send the file back and forth___
The game goes by the two players IMing or Chatting or Emailing their moves to one another. i.e-- "Marine to K-8" or "Tank -attacks- Jeep @ E-2"
The best way I can think of to do this, is to use IRC, I remember playing DnD online with just an IRC chat room and a "Dice-Bot" Handy little things. Basically you just say a command like "Roll 2 d6" and it will give you the result of rolling two six-sided-dice, or "Roll 3 d4" etc etc. I think that would be the best way to do it.
Sounds like a good idea, not sure if anyone would really be willing to put in the grunt work of getting it in motion though. =/
But the prospect of using units (and hero-units?) that you concept and craft yourself is pretty awesome.
Crits, Advice, and Feedback always appreciated.
"There's smoke in my iris, but I painted a sunny day on the inside of my eyelids" -Aesop Rock