Minimum chainring sizes for 130 and 110 were mentioned. If other BCD's are allowed, the question becomes derailer capacity --and that is a function of derailer design. I'm currently experimenting with a 44-28 (no ramps or pins to aid the climb), and use the IRD compact road triple derailer. It works absolutely fine, BUT I've also installed a chain catcher at the low end and a chain guard at the high side. Shifts appear to be a simple slap on the brifters. I have not been on a long trip yet --which is where the real behavior will emerge. But it seems to be fine and I suspect 44 - 26 would also.

[adding: this derailer appears to be able to handle up to 48 big ring.]
[adding pic]



[adding: not sure I'd want a 48 - 26, but I bet this derailer could handle it.]