My word, but you are quick — and very good at explaining the problem in terms that I can understand! Kate (Amythestfenix) will vouch for the fact that I’ve never done much with meshes, and know squat about making one.
I see the problem with that little window. It would have been very handy if slaving it to the double-hung window worked well, but the mapping is not very nice. Here’s an in-game test picture of the slaved version you sent:

If I understand you correctly, the other version, simply recolorable, would result in flat color without wood graining. That would be fine with me — I can pretend the window has flat aluminum siding!
In the event that you would be willing to add some other colors, I wouldn't need many. Maybe a medium brown like the one that comes with the Maxis Independent Expressions, and a black one.
In any event, I am eternally grateful for your help! I’ve long admired your work and am in total awe of the talent that you, Lunie, Kate, and others here at Sims2Artists and at MTS share every day!

so very much!
—Lightheart, aka Betsy