Did some measuring with various wheels and tires. Bottom line is that folks are correct, the rear isn't going to take more than a true 25mm, while you have more room in the front and could easily get a 28mm tire there.

Front:
23mm Conti GP4000 II on Araya CTL-370, measuring an actual 23.5mm wide - a good 8mm clearance side to side, and plenty of room above.
25mm Conti GP4000II on Flo30 rim, measuring an actual 28.5mm wide - 5mm clearance side to side, and plenty of room above (brake caliper was within about 4 mm).

Rear:
23 mm Schwalbe Lugano on Ambrosio 19 Extra Super Elite, meas. actual 23mm - 7mm clearance to each chain stay, but maybe 3mm to brake bridge.
25mm Conti GP4000II on Araya CTL-370, measuring actual 26.5mm - 4-5mm to each chain stay, but just rubs on the brake bridge.

It appears to me that you could grind down the bottom of the brake bridge by a couple of mm without any worry, but that's not going to get you much. As I noted before, the same tire will have different widths and different heights with different rims, by as much as a couple of mm. And of course many tires don't measure as wide as they claim, and a few measure wider than they claim.

Bottom line - if you don't file the bottom of the brake bridge, you're stuck at a true 25mm tire in the rear, and even if you file it you aren't gaining much. I think the 650 conversion that's been suggested is the direction to go if you want some real width in the back. Since the front can handle a 28mm tire with no problem, I wonder if just doing the 650c conversion in the rear would be an inelegant but less expensive solution?