it's impossible with the technology today.
However, if you were to start your own rules, you could require a voltage limiter to bring a LiPo pack to 4 cell voltage.
you could also start LiPo spec class with the Novak Crawler motors or lath motors (both high turn) and a standard cell. Or find an airplane/helicoptor motor that is smaller than 540 size but bigger than the 1/18 brushless and run those in a spec class. With a 3000-4000 mah batt, you would probably be looking at a 30 minute main with any of the three
You can't technically have 4-cell racing with LiPo, but you could synthasize your own rules to use them. Of course this will leave someone out in the cold, so you will probably be doing what you are doing for a while yet until a battery company builds their own "spec cells" with the regulater integrated.