My gut reaction was: no way is this going to happen, mainly because people who mine usually want the Bitcoins for themselves. On further reflection, I think this isn't quite impossible to get working - maybe in the TOR userbase there are people who'd be willing to part with some GPU cycles to support the system, even if they wouldn't be interested in mining as such. I think you'd have to make it voluntary, though, with the TOR installer / executable asking the user if they're willing to mine, and my feeling is people tend to say no when they're asked to do something extra.

In other words, there may well be a bunch of people who'd go for this, but whether it's big enough to justify the effort, I don't know. The biggest problem is, you'd probably have to get it included into the TOR client, which is likely to be an uphill battle as it would probably seem somewhat tangential to their actual focus.