As a suggestion, would it not be worth taking the time to go through the email templates, finding out what variables they use (not just what they claim to use, but what the source actually exposes to the email loader), and providing that as a list to users?
I've been doing that today and I've gone through all the templates, figuring out what's available, it's pretty dull to backtrace everything though

I did also harmonise some stuff, for consistency as well as provide a description to users of when each email will be sent.
When I first saw the mod, I actually thought the list on the left were actually a set of buttons that one could click to have the variable inserted directly into the body, like bbcode etc. is in the main editor normally. Might be another nice feature to add.
It certainly would be a great addition to 2.1 and I see no reason at all not to include it as a core feature (you're a developer, just implement it and worry about complaints later, hahah). I'd also be inclined to move it to Admin > Maintenance > Mail as a subitem rather than in the main area, though. Keeps all the mail stuff together.
If it does get added, spending the extra day to collate all the information is really worth it IMHO.(Screenshot attached for what it can look like. I suck at UI.)