Looks like it has been a while since your post, but just in case you or someone else checks...
I am 39 years old and have recently been having problems with my right index finger and middle finger (the one between my index finger and my ring finger) twitching while I am using my mouse. As you described, it only happens when my hand is resting idle on the mouse. It is especially annoying because whenever it happens, my finger hits a mouse button and I end up doing something I didn't mean to!
I am a software engineer, so as you can imagine, I use a mouse a lot too. Although I occasionally have mild symptioms of CTS, I don't think that is causing this problem.
In addition to the twitching in my hand, I have been noticing small twitches in my arms,legs and feet, and they always happen when the muscles involved are at rest. Also, there were several consecutive nights/mornings about 2 months ago when I was experiencing much more noticeable, shocklike jerks of varying severity. Sometimes my shoulder, arm or leg would extend suddenly, and a few times my whole body was involved and would pop right off he bed.
After I told my GP about these more severe incedents, he referred me to a nuerologist who ordered an EEG.
During the EEG I noticed I was having the same kind of mild twitching that I have when using my mouse, and now the neurologist tells me that the EEG shows abnormal electrical activity. I am pretty convinced that the EEG abnormalities are related to the twitching I was feeling during the EEG. I had an EEG 2 years ago (before the twitching started) and it was completely normal.
The neurologist has not determined what is going on yet with the abonormal EEG, but in general he seems to think that I am having epileptic seizures. Although I was referred to the neurologist because of the muscle jerks, he actually ordered the EEG because he believed, based on my lifelong medical and school history, that I might have an undiagnosed case of absence (petit mal) epilepsy.
Anyway, my point is that you might want to consider whether your history includes any evidence of epilepsy (most likely childhood absence/petit mal seizures that were mistaken for inattentiveness). If it does, maybe your finger twitch is related to that and could be controlled by anticonvulsant meds.
My neurologist has referred me to an epileptologist for further investigation of the EEG abnormalities. Once that is complete, I will post the findings here for anyone who is interested...