Well, I had a go and it hasn't gone too badly. But it'll depend on how well it lasts as to whether it is a success or not. I've done this before the heel rubber is excessively worn, as once it gets too thin it can often give way even before it has worn fully through.
Firstly I trimmed and ground down the worst worn of the heel grips to make a mostly flat surface, then cut a rubber patch and glued it on with Klebfest contact adhesive (apply to both sides, allow to go tacky and stick together):
The heel at the very back of the other boot had worn somewhat thinner though, leaving a curve; I'd hoped the glue would hold the patch to this curve, but after about a day it gave way of its own accord and the patch sprang back to being flat, which perhaps isn't too encouraging for the bond strength. So I got some shoe-goo and, after cleaning out the gap, filled it.
The shoe goo is very sticky stuff and some people even use it directly to build up the heel; I shall possibly have a go with that in future as an alternative to using a rubber patch. But the goo is reasonably expensive and apparently doesn't keep too well once the tubes are open either, so rubber patches would probably still be cheaper 'if' this works. Next time though, I would build up the curved bit at the back with a separate bit of rubber, to make a completely flat surface before sticking on the patch.
So I'll be testing it over the coming weeks, when doing local walks where failure wouldn't be too awkward. Fingers crossed!