You can, of course, pose the mannequins, then create an adaptive skin based on them, and sculpt that… Although I doubt that posing will be the long pole in the tent, time wise. Since you’re doing just one pose, it’s probably best to just use the transpose tools to force your base mesh into position (assuming you have a base mesh – if you don’t and your characters are female, you can use the one that I’ve posted. EOF also has one that is better. There’s a Male one also but I don’t remember who posted that.).
posing a nearly-finished base mesh itself will be a lot faster for you – just fix the glitches from limb bends, and sculpt your clothing on top, and polypaint. You could also use the mannequins to rig the base mesh, but that won’t save you any scultping time, and the posing time will be a negligible time savings.
Will be mighty impressive if you can go from unposed base mesh to posed, clothed, and painted product in the time you’ve got alloted… Good luck!