Though likely we'd still use the market rate and not a fixed SBD amount

Why bother? The goal I presume is to burn "some" Steem to help overall Steem economics (the exact amount doesn't matter) as well as serving as spam filter against frivolous submissions wasting the committee's time (again, the exact amount doesn't matter).

A simple requirement to burn exactly 100 SBD does both and is easier for both the submitter (to know how much to burn) and the committee (to check that the correct amount was burned).

The only situation where this is a problem would be if SBD goes on a big pump again, in which case it is better to not burn SBD at all and switch to STEEM. But we don't know if or when that will happen. It may be a long time or never.