I just found this interesting option for a chair wheel.
http://www.cokertire.com/480-10-bf-goodrich-blackwall.html
It's section width properly fits within the 125mm limit in the rules as written. I wonder if it is any better than a trailer tire but I do note that it is for classic Mini's so it should be able to handle a side load. Interestingly, it only requires a 3" wide rim. Hmmm ... are there are 10" trailer rims only 3" wide? The early Mini steel wheel was 3.5" wide so it could be used with this tire. Alternatively, a similar tire is available in 12" and trailer rims that size are readily available.
They also sell a 16" tire that only requires a 3" rim. A little wider than the Dunlop R2's but within the section width limit of the existing VRRA rules.
http://www.cokertire.com/450-475-16-firestone-blackwall.html
A skinny bias-ply tire is not confidence inspiring, but that's exactly what the Dunlop R2 is (and when was the last time you raced in snow!)
Coker Tire has several vintage tires that might be interesting. If someone wanted to try radial tires on 15' rims, this would be an option and the section width is within the rules.
http://www.cokertire.com/125r15-michelin-x.html