Thank you for your prompt response.
I'll just have to keep the ignite trigger then, and then add a second trigger that is filtered to only prop_ragdoll that entity-kills !activator after X seconds. This will also burn mattresses if I add them in a later release, but that is so incredibly awesome that I could care less.
Yes, I know I could name all the mattresses "mattress", then exclude them with a filter_multi, but where's the fun in that?