The gear down long range cruise altitude capability tables for my 737 go up to a pressure altitude of 39700. The main concern here is residual climb capability of 100 fpm at that altitude with max cruise thrust. There is no altitude limit for gear extension, just a speed limit of M .82 and 320kias once its extended. Fuel flow tables only go up as far as FL370 though.
Nothing special required for gear down ferry flights, any normal line pilot can do that, just get out the QRH and check the fuel tables. FMC fuel predictions can not be used though.