Hi Leith, did you ever hear back on any alternative solution that worked better? I'm in the exact same boat right now (a many-to-many relationship that should use CONTAINS for the conditional instead of =) and in our case, your imperfect solution might not work (the end user would not be able to work with this). In our case, if we can't control the input properly, we might have to just allow people to pick from the large list and then do data sanity check to prevent incorrect combinations, but it would be awesome if there was a way to filter the drop down from the start!
