yer good topic, im not sure what the answer is, but there is some good discussion here, and id love to hear how the tactic of 'letting coach decide' works out.
I dont think you will ever be able to replicate the scenario of some NBA players, who, play 30+ mins but come off the bench. Starters who only log <10 mins, i just dont think its possible to do in this game.
Setting one player as backup to two positions is a good way to get more mins, but you want more still, i understand that as well and i agree with this.
an interesting tactic, might be to play a guy with low stamina as a starter? My C has/had pitiful Stamina, and he seemed to only play 25-30-ish mins per game. unless i set him as backup as well, that increased his mins a bit.
So maybe... get a starter who has great stats, but has low stamina? And set ur 6th man to replace him. That might help.
The other thing that ive started doing with a good 6th man, is have your 6th man be an 'out of position' player. eg: Buy a SG with a level of ID which doesn't hurt you too much, and play him at PF? its only for 10- mins a game, and dependant upon your tactic, you might get more offensive punch from the out of position guy. I know this doesnt increase his minutes, but it may increse the 6th mans offensive output?
worth a try in some PL or scrimmage games anyways.