If you don't want to deal with TagTime (and I don't blame you) then just hooking up RescueTime to Beeminder is a great way to go. Here's what RescueTime says about Beeminder:
I am going to add this feature to a new version of my Productivity Owl:
Even if a Chrome Extension isn't what you're looking for - thanks for this idea. The way my owl works is that you schedule freetime, and during non-freetime all pages have a timer, and when the timer reaches 0, the owl closes your tab. It also has a block list for time-wasting pages like reddit or HN.
Lately my problem is that I delete the owl's timer, or schedule freetime when I shouldn't to avoid doing work. I was going to implement a "Respect Score" which I'm still planning on doing - but I really like the idea of the owl recognizing my 'workarounds', and prompting me with questions about my goals, instead of just berating me. Maybe I'll let users add their own prompts.
So I think your idea is a perfect addition to my current system. I plan to have a new version in 2-3 weeks.
What if the user had to enter a predetermined answer to the question prompt in order to continue? By making the user actually type something that they themselves chose the prompt becomes more powerful.