http://www.aopa.org/asf/publications/sa03.pdf

AOPA's take on the BFR. Haven't read it all yet, but the reg does say "FAR Part 121 Pilot Proficiency Check" as something that can take the place of the BFR. Doesn't specify it has to be a PIC check. The BFR applies to exercising your privledges as pilot-in-command under any part. Its not just a 91 operations issues. There are more stringent requirements for 121 and 135 while supersede the BFR.

Edit: Ok, I read too fast. I just says "passed a pilot proficiency check conducted by an examiner, an approved pilot check airman". Still doesn't distinguish between SIC and PIC checks.