Can you apply plug-ins like Ambience Reverb to MIDI tracks? SO far i've thought the only way you can do this is after expotring to .mp3, which has worked fine, but if you could do this it would be helpful in my non-orchestral stuff.
No. There are generic midi effects, one of them being reverb, but they generally aren't any good. The only way around this is to find a VSTi that lets you use soundfonts and then you can apply reverb to them. There are programs like this out there.