Is it possible to obtain an EntityType (or its int value) for the PK or FK object in an IEntityRelation?

Could you please describe the context in which you want to use it? Perhaps there's an alternative to what you want (as the entitytype isnt directly available)

I'm creating a dynamic filter engine. I would like to allow a user to choose a relation, then pick fields that belong to the entity on the other side of that relation.

The best way to do this I think is to pre-generate code / data from the project. You can do this by writing some templates which produce code you need / data you can use at runtime for the purpose of your generic search engine. With a few template statements you can already produce the data you need I think.