I noticed one of the Ebay auctions I mentioned earlier for a 1x 24W:
"LUTRON ECO-T524 T5 24W ELEC. DIMMING BALLAST +ENCLOSURE"
Is for a single quantity only, but in the description it says that there's more available. $16.88 Buy it Now, and the seller is also taking best offers. So if you don't mind having two ballasts, one for each bulb, chances are you can contact that seller directly and he'll probably deal with you on the side, or start a new auction for quantity two. That would set you up immediately rather than having to wait and watch, and still at a decent price.
A microcontroller would be optional. For interfacing purposes, a "control voltage" type ballast would be easier, since you just control it with PWM through a level shifter or optocoupler. The ballast will have an internal RC filter with a fairly long time constant, typically about one second, that will convert even low frequency PWM to a steady DC.
"Three wire" is a little tougher. For that you'd need to create a microprocessor controlled AC dimmer. I've got a PIC24 on my desk right now which I've been playing with. Eventually I'd like to integrate one into my hood, along with Zigbee RF control or similar.
Now if you instead go with building
your ballast from scratch, you will need either a microcontroller or a specialized IC.
The specialized ICs may be hard to get, things of that nature tend not be available to anyone but serious OEMs. But I haven't looked into it directly.
Microprocessor-based reference designs are also an option. But make sure you fairly evaluate your existing skills, and the time needed to learn new ones. You don't want to blow your capstone deadline! Perhaps you could adapt a reference design like this one:
This is for a programmed-start, 1x 32W T8 dimming ballast. Given that I have successfully driven T5HO with a T8 dimming ballast, it would probably also work with 1x 24W T5HO. The most critical change would be proper end-of-life detection for safety, which was lacking in my instant-start T8 ballast experiment.
As long as you have a microprocessor controlling either a consumer ballast, or at the heart of a custom ballast, adding a room light sensor is trivial and inexpensive.
Just some ideas that may help you get started.