A more complicated system would allow for some automated conversion between the two coins and all the proposals would compete with each other, regardless of the coin they were denominated in

Although this seems to be off the table and not needed, I don't think it would be very hard to: a) keep the fund in STEEM, b) make the optional conversion from STEEM to SBD at time of payout based on a property of the worker. I.e. the same way content rewards currently work. The refund worker would be required to be STEEM-denominated in this model.