Hello there!
On the compatibility of BESM d20 with other d20 products, I have a more precise question: are you going to mix skill or just renaming them?
i.e. you could potentially create a stealth skill that include move silently and hide in shadows, so its opposed skill would be both spot and listen if you use a character from a different system as an opponent.
I don't have SAS, but thing like that might make the system completly incompatible with other d20 sytem product. There might be a few skills in it that made people angry.
Also who are the authors? Are they anime/mecha expert, d20 expert (not necessarily in number of product, but in quality), both?
DP9 plans to release a mecha compendium, are you working with them to great length, or is it just allowing them to see the draft copy of d20 mecha?
Is d20 Mecha playtested at Pyramid/d20weekly? do you hire french playtester?(like me

)
I think this is all, unless you want more questions?