Ed, I have 2.8 tires on wide (34i) rims on my 56/700c Susie and am trying to figure out fenders too. I just tried a set of SKS Bluemel 75s in case this experience helps:
There is plenty of frame clearance, even with a good gap to the tire. As seems to be the case with many Rivendells, the tightest spot was at the bottom of the fork crown - not at the seat tube or chainstays or braces, as you'd expect with other bikes. I don't think I would have even needed to notch the rear for chain clearance, but that's a function of bottom bracket spindle length so YMMV. However, I did have interference at the cantilever straddle wire. (I used cantis instead of v brakes specifically because I wanted to run big tires and fenders, which wouldn't fit under V-brakes.)
I could solve this and make these fenders work by raising the straddle wire a tiny bit, but I'm reluctant to becasue I think I have them adjusted pretty well for power. Or I could notch the edge of the fender to allow the cable to cut through, but that seems dangerous. A different canti arm profile might help too. (I'm using the Dia Compe 980.) I was going to try an extra-wide straddle-wire hanger - which would proably solve it - but I kind of decided that the black fenders don't look so good on the bike. If you have a different color scheme than me though and don't mind black, this may be a pretty good solution. They're nice looking and nicely-made fenders - just not like the silver SKS that I'm accustomed to.
My next step is going to be to try the Simworks/Honjo Flat 80 fender, but they're super expensive so I need to make as many measurements as possible first. The advantage of using aluminum will be that I can deform them to fit the fork crown better than plastic ones, where the critical clearance point would otherwise be as noted above.
Last option will be to use some Planet Bike 60mm fenders, even though those wouldn't provide full coverage.
Boils down to a lot of trial and error unfortunately, but I can at least confirm that it will be possible in one way or another. I'm going to put the Bluemel 75 fenders on another bike soon but haven't yet, in case you'd like me to take some photos of it mocked up.