All good creative inputs guys. I think Kevlar is intending to try using some kind of hydraulic hose, stretched over a wheel. If it can be done and it doesn't crush the wheel, I'd say it's got real potential. Sorry if you were hoping to keep that secret, Kev. Unfortunately, there are a gazillion different rubbers and synthetics so the chances that the formulation for a hydraulic hose is good for tyres is pretty remote but it's worth a try.
Nic Case, current RC World Speed Record holder has put a lot of time and effort into moulding tyres onto wheels and vulcanising them on the wheels themselves. I think he would admit that his results have been inconsistent so far.
I haven't been into it in any great depth as I elected to go the foam route. I used a set of Jaco foams that were originally produced for the Savage....
I needed to make up adaptors to suit the 14mm hexes...


...and in fact, they worked very well indeed. As you can see, I narrowed the front wheels by about 0.5". If anybody fancies these, together with the adaptors, send me a PM as they're just drying out in my workshop.
There was another guy who persuaded John's BSR to make up a set of big foams for his E-Maxx. They were narrower than mine and cost him a whole lot without much success.