I may be completely wrong, but I think you'll have a hard time convincing an authorized retrofitter to actually put S&S couplers on a BMX frame -- especially if you tell them you plan to grind with it. I'd think you'd slam the couplers constantly when grinding, and eventually screw them up.

I do not see any listed on the S&S site (http://www.sandsmachine.com/), in fact there are very few MTB's listed.

Also FYI, you can't retrofit aluminum, which I'd assume makes up most (but not all) BMX frames.