I think you misunderstand my suggestion. I'm talking about choice per registration. I.e. all long sleeved or all short sleeved, not race by race choice. I suppose that some might want a long sleeve in some an short sleeve in others, but I would think that number would be FAR SMALLER than those that would prefer all of one or the other. Obviously some special shirt designs (i.e. the Marathon with the park logos) would have to all be long sleeved. I'm sure they fill them in an assembly line sort of way. I.e. the setup the boxes in a line for one particular size (say XL), then grab a bag and walk down the row of boxes grabbing each of the appropriate shirts (i.e. one of each for Dopey), then put their bag in a pile and start over. They repeat the process until they have X bags filled (whatever X is for that race/challenge). Then the move onto the next size. What I'm proposing would add one extra step. (i.e. after they fill Y bags of LS shirts), they change the LS boxes for SS boxes (this is the extra step) and then fill up Z bags of SS shirts, and put those is a separate pile. Note that Y+Z = X so the number of total passes would be identical. The only difference is changing the LS boxes for SS boxes at the appropriate time, which is a minimal time addition. Then at the Expo, when they go to grab your shirts out of the appropriate Size Pile, they have to either grab from the LS pile or SS pile as appropriate.
True that they accept registrations all the way up until the race, but the vast majorities will be placed well in advance. Plus they could easily add a requirement that registrations must be received by date X to guarantee size and style (like most races do).
I don't either, but considering that WDW races are 3x-4x the cost of other races I have higher expectations.