I don't have a definitive answer just a similar problem.
The brakes on my 1988 Schwinn Voyageur don't work as well as I would hope after I changed the wheel size down to 700c. I believe these brakes to be the original brakes. They were on the bike when I got it. It was in really bad shape and reusing the 27" wheels were not an option. The main issue I have is with the front wheel. The brake pads are at the wrong angle. I've thought about grinding them down at the correct angle to let more of the pad engage the braking surface. Obviously this will GREATLY reduce the life of the pad. I haven't done it yet since I have a build in mind and I'm a few steps away from completing that build. Also the way the brakes are set up currently they work well enough to stop under regular riding circumstances. But braking while fully loaded with panniers and/or in the wet, probably not so much. The rear brake pads are aligned correctly but I can't get an inflated tire between the pads without really helping it in there. The profile of the current canti brakes on both front and rear are very outboard with doesn't allow me to use the current racks and panniers I have. A thinner profile, which the above picture shows, would hopefully allow me to not have to buy another set of racks.
I've read that v-brakes will quickly solve the spacing issue. The problem I have, is v-brakes are ugly. Appearances are important to me.
On a different thread dealing with the same conversion compatibility issues, I also read a new product could work because of its design. They're the Shimano BR-CX50 canti brakes. Not too expensive if they do solve the conversion issue. I plan to buy some of these at least for fork. If I do it before you I'll post results.
It seems as if there could be a little adaptability if need be. I've seen some concave and convex washers that happen to be different widths. I would imagine fiddling around with different widths could result in different pad placement. It also looks like the 'washer/round spacer' between the pad and the rest of the brake could be filed down to a thinner width. I would try it. I've taken some liberties with some of my bikes to end up with a desired result that I wouldn't recommend to others.
I have thought about lacing some wheels together with 27" rims. That would solve all my issues. As I said earlier the original wheelset was trash when I got it. I would have been seriously polishing a turd had I spent any time to repair. The main issue I have with this solution is tire options. There aren't as many tires for a 27" wheel. Plus I would have to lace them together which is not something I'm too familiar with. I know the gist of how to do it but applying the knowledge is something different. So then that gets into paying a good amount of money to lace together a wheel set.
I've also thought about having the canti posts moved/replaced by a frame builder. This could be a good solution to a frame like mine, since the paint is not in good shape. Which could lead into getting it painted or powder coated in a color I like more. But a frame that is pristine, or where no replacement decals are available, possibly not.
I know a professional bike mechanic who has a late '80s Trek 520 with cantis who's tried many different options to set up with 700c wheels and failed at all of them. I don't believe he has tried the Shimano brakes I posted a pic of. He got defeated and put the 27" wheels back on. If I remember correctly, he's just going to lace together a new wheelset sometime in the future.
Hope some of that helps