We would have to build/develop small trees if we're working within a ten-year timeframe, and they'll still have some wounds in need of further healing at the end of the contest, but a fifteen- to twenty-year timeframe just seems too long for a contest (although I am doing a twenty-seven-year nebari challenge as a bit). It should be doable to develop a tree in ten years, even if it isn't highly refined and ready for the National Show.
Like the six-year JBP contest, I think the idea would be to push to see what we can accomplish in a timeframe that's very short for deciduous trees. Ten years from seed seems like the shortest plausible timeframe.
I plan to sow about five pounds of acorns this fall. Healthy acorns have a very high germination rate, so I hope to have an enormous number of oak trees. I'll select the ones I like best for this contest and develop them for a few years into pre-bonsai. From there, I'll whittle it down to a couple of trees to be likely final contest submissions, and I'll work on developing them from pre-bonsai into bonsai. I'll probably continue growing the other choice trees into larger pre-bonsai for my own enjoyment outside the contest.