ShowRSS right now works as a piece in the workflow for many people: feed what aired a few hours ago to another system. It hasn't got an archive and it doesn't serve as one. Thus, no option to do what you say… it's meant for the last week or so, not a lot more.

It does what it does pretty well, though. The new system reads a few sources (not just EZTV) and picks the best releases. So, 720p for everyone in a matter of hours and for almost every show you can think of.

P.S: have you thought about using transmission+dyndns? It's how I run it (minidlna+transmission, and showRSS pushes to my rasppi, no RSS involved!).