Is it possible, or will it be in the future, for custom signature and avatar backgrounds to be different for one character? Say I use my custom background on my signature for Jhaeldril, but on another forum, I want to use a different background on an avatar for Jhaeldril. Would that be possible?

As it stands now, there is only one custom background per character. So, if I have a custom avatar background, and try to use that on a signature...300 or so pixels are blank. =(

Currently, no

The next major version of SigGen (1.0.0) you will be able to do this
In fact, it should be able to provide multiple "themes" that you can select from so you can use a different sig/avatar for each forum, or mood

Awesome Zanix. Let me know if you need any help with testing or what not. I enjoy messing with the sigs. =)