Personally, I think it would be interesting if FFL's stopped processing certain transactions as well, i.e. LE-exempt transactions. That would add a little more sting to the effort. I don't see an Equal Protection issue, not a Federal Civil Rights Act issue. There might be something buried in ATF regulations, or maybe in various state statutes (CA may have one) about having to process certain types of transactions, so more research would be required.
That would either 1) dry up the supply to LE (slightly) or 2) force LE to engage in certain questional transfers (illegal ones, I mean).
If we're going to do this, it should be all-in.