different number of posts by RSS feed (in WordPress)

I’ve done a bunch of searching on this site, WordPress.com, and Google, and I haven’t found an answer, so I’ll try posting here! If it’s already been answered, I apologize in advance for my search-fu failure.

I run a podcast and blog using a WordPress/PowerPress combo. Because of the limit on many aggregators, I have to keep my site’s main feed under 512kb, so I’ve limited the number of posts that show in RSS to 50. However, we frequently get people asking for a feed that has our entire catalog so they can pull down our show’s full history.

Is there a plugin or some feature in WordPress that would allow me to leave my main feed with its length restrictions but launch a parallel feed that shows everything? Or is there a relatively simple trick I can do in the PHP?

Thanks in advance for any help!