NP, actually strike all of this: "sequence in the block version field, with each of the first 29 bits each representing support for a given BIP." (not in block version field, not 29 bits, not limited to BIPs).
This was a mental error of mine. I somehow thought Libbitcoin used the same bits in the rule_fork enum as used in the signalling during BIP activation. I now realise this is not true, bip112/113 used bit 0 for signalling, but occupy bits 9 and 10 in Libbitcoin, for example.