I would have to suggest GHS Brite flats. They are ground down roudwounds. They have the feel of flats but with a bit more edge to them. You can get them from Link Removed for a pretty good price. Other than those, I'd also recommend D'Addario Chromes in a light gauge.
I have the Brite Flats on my Precision, and the D and G strings are definitely brighter than regular flats, but the A and E sound pretty close to flats IMO. At first, they almost sounded like two different string types, but the gap has narrowed as they've broken in. I do like them, but prefer Chromes, which are definitely brighter but still sound like flats, and GHS Precision flats, which provide delicious old-school phatness.
FWIW, I tried D'Addario half rounds, and did not like them at all. My set was a fairly heavy gauge (I think the 55-110), and they were brutally tense and stiff. They might be worth a try in a lighter gauge, but from my brief stint they didn't seem much brighter than Chromes.
