Yeah, you've answered your own question correctly.
There's no way to do this safely on a per-session basis. But I'd worry about that regardless... By definition the result mappings should be deterministic and consistent. They shouldn't really change on a per-request basis.
That said, if auto-mapping is enabled, and you simply leave out the columns that you don't want mapped (using dynamic SQL), that might work.... but it won't be pretty.
Cheers,
Clinton