for the past 12 years i've been running mtb 26" (559mm) rims on my triathlon custom bike originally designed for narrow 650c rims. no problem.

i found the narrowest, least expensive, silver, 28h, in 559mm (mtb) diameter i could find. turned out to be two sun 0 degree 28h rims for like 12 apiece on ebay. then i found a set of 28h hubs in correct width (130mm) and drilling (28h) and bought some super light 15g-18g db wheelsmith spokes and some alloy nipples. slapped them together with a pair of 25mm slicks. you can make up some really light mtb wheels. the parts are out there, but it may be difficult to find a set already made up.

maybe if you buy the parts and talk to the LBS about your project beforehand, they may agree to build them.

BTW, most major bike tire makers have one mtb tire in 25mm width. you can try the wheelchair web sites too. just beware, their sizing descriptions are a little, let's say, different?