Currently the add-on doesn't work unless rich formatting is selected in the compose mail frame.  Can you make an option to force insertion of the signature even if rich text formatting is not enabled (plain text formatting is enabled).  Since gmail does a nice job of converting the rich text to plain text maybe you could just enable rich text, insert the signature, and then convert back to plain text.

I noticed that the options for the signatures only come up if the user is in Rich Formatting mode. Perhaps there could be two different interfaces for the two different modes. I wouldn't mind if my signatures for plain text didn't carry over to the rich formatting, so you wouldn't have to worry about stripping/adding html.

If you're uncomfortable with having two sets of signatures depending on what formatting mode you're in, perhaps you could release the plain text version as a separate add-on. Those of us who work only in plain text could just install that add-on. I would suggest a fork in the project, but the code isn't open source .