I'll be shooting a concert and I'm not sure if I'll be permitted to bring an external mic. Does anyone know of any post filters to make the audio from the 7D sound "fuller" (not quite sure what term to use)

Brandon, try the app approach more than the filter one. On what application are you working on your sound?
When working short of time I use Adobe's Soundbooth (since I'm also using Premiere and/or After Effects). It has this "Effects" tab that has more than enough sound twitching engines for any effect you want to give your soundtrack. In that tab try the "Dynamics/ Better Presence - Aggressive" Preset. Should do the trick.
When I want to squeeze the max on a soundtrack I tend to use Bias's Peak Pro, but it takes much more time and the mastering process is lots more involved but you are in total control.

Brandon, if it's a fire department / audio cable issue perhaps you can find a way to use a wireless system and put the lavaliere mic on a nearby speaker. I've had to do this countless times - sometimes surreptitiously to get adequate sound. Basically you are F$#%ed if you have to go with camera mic sound. I'd be curious to hear if there is a magic bullet for camera mic sound as I have some older tapes with just that problem I'd love to process out better sound on. Then there is the concept of double system sound where you would place a separate recorder at the mixing panel or even next to a good speaker and go with PluralEyes or something like it to sync the camera mic sound to the external recorder sound.

It would be best to have a separate recorder anyway. You could let it run continuously, while the camera will have to stop and start periodically. Hopefully you can get some wide shots to repeat to cover the lapses, or use a second camera started up a minute later than the first.