Variant applied to HEAD: schema_init.c 1.473. Correct 'match' code is simpler than yours, now that you've pointed me in the right direction.
All ordering rules except CSNOrderingMatch and UUIDOrderingMatch now support extensible match. Someone who knows slapd's use of CSNs and UUIDs can (tell me to) do the same there if it's OK to do so. I didn't want to meddle carelessly, also I don't know what UUIDOrderingMatch's SLAP_MR_MUTATION_NORMALIZER flag means and if it would interfere.