I've seen 27" to 700c conversions and 26" (559mm) to 700c conversions, but I haven't seen a (non-disc) 700c to 26"(559mm) conversion, and am not sure it's feasible without modifying the frame (install canti studs or disc mounts).

Looking at the wheel diameters I do have an off the wall question of my own; if you have a bike made for 26" wheels with u-brakes and convert to cantis or Vs could you use 28"(635mm) wheels?
The size difference would be 38mm, with u-brakes reaching down 20mm and cantis extending up 20mm.
I know it's not a common wheel size, but does anyone have them? How much clearance would the frame need to have (with the skinniest available tires)?