Have you done this, or just read about it, or is it just a cunning idea? bitd these were around but fitted to a non-guarded wheel secured by the rim brake bolt and nut (at fork crown/seatstay bridge). Shown to have no anti-puncture merit (which is why they became extinct).
Have you tried to fit "smaller wheels" to a bike, or just read about it? If 'yes' tell us how it went.
I used to use tyresavers which generally fitted to the brake bolt, being of a finer wire than coathanger wire. The wire that fitted to the brake bolt
was connected to the wire that rested across the tyre by a fine piece of clear plastic tube each side so the wire could rise or fall with any imperfections on the tyre surface. I wore a few of them out over the years as the abrasive grit that was carried round by the tyre wore away at the wire where it rested on the tyre. I was able to fit them under the mudguard at the brake bolt because I had enough clearance, unlike the OP.
I never needed to try alternative fittings such as at the ends of the mudguards. They did need rechecking or adjusting from time to time to make sure they were still resting on the tyre centrally. I have seen thorns and flints flicked out by the tyresavers so they weren't completely useless but they weren't a 100% deterrent.
Although not strictly relevant to the OP's situation I have changed from 27 X1 1/4 wheels to 700C wheels in search of a better tyre choice when the former were becoming less common on new bikes. Mind you, that was back in 1983 when I was having my frame refurbished and components upgraded in any case. The result, for me at least, was highly satisfactory.I have rarely had issues with mud clogging mudguards (and with that particular bike, the increased clearance inside the mudguards after changing the wheels made that even more unlikely.) I have had mudguards clogged with snow, however. The only solution to that is probably, no mudguards.
More recently, I did consider for a while seeing if it was possible to fit 26" wheels to my 700C disc braked tourer as due to the discs, swopping them would have been easier. I'd read about it being done on the Cycling UK forum. I might also have to consider such things as shorter cranks to compensate for the (relatively small) drop in BB height, though just riding it with lower cornering clearance in mind would also have worked, and slightly lower gearing due to the smaller wheels. With the same cassette on the back, it might even have been possible to swap the wheels over summer and winter.
In the end, apart from a slightly lower stand over height and maybe handier handling in tight spaces, the cost/ benefit ratio wasn't enough of an incentive over my 700C set up to justify it when it's a nicely balanced bike and fulfilled 95% of my cycling needs. The thought resurfaced a few times over the years and then along came recumbents into my life which put everything into a whole new perspective.
Generally speaking, whatever my tyres, I like mudguards.