(for pure speculation and reflection)
I think sgreng-ba being the active purposeful form of 'greng-ba, it must mean "to make a horn to rise up." Or maybe just "rising horn," which could describe a geographic feature of the area., like a mountain peak or something. At same time, I am in no position to argue against Ulrike's argument that it means 'perpetual [water] spring.' There is a holy spring there that the pilgrims are supposed to go to, so maybe that could explain why local people would want to associate the name with the spring? I don't know.
Dear Dan, yes I think one would normally think along what you suggest. But (as a Tibetan saying goes) if I have not been to lHa-sa myself, you can tell me anything about the place. :-) D.
Sigh... what you say is soooo true. We went past both Reting and Taglung, seeing them from a distance, but didn't go to look closer, thinking there would be a next time...