sounds crap when ds1 was potty training (same age) his nursery did exactly as i asked them to... put potty in view and asked him every half hour or so, plus helped him when he wanted to go (he couldnt do his own trousers then)
If the nursery are going to be so silly about not taking him to the potty I would send him in with pants on with pull ups over the top.
That way your son will know when he has wet himself but you wont have loads of trousers to wash - just the pants. If you send him in with just pull ups it will take longer for him to realise he is wet and the nursery will be less bothered about asking if he needs to go as it's "safe" as he's in pull ups.
He will/may fall out of the habit of asking or lean to become "lazy" about asking to go in nursery. Going for a wee is very distracting when you're in nursery as there are loads of other much cooler things to be doing so he needs to get into the habit of tearing himself away from play. Wet pants is probably the only way he will earn to ask if the nusery wont "force" him. But you need to be confident he will know he is wet - otherwise the chances are he'll be in wet pants for a good few hours which could make him sore.
Sorry they sound nuts! I'm a nursery manager, can understand their H & S concerns (well not actually but trying to be generous!), why can't they keep a potty to hand and then CLEAN it straight away? And as for not infringing his rights, blimey how on earth do they cope with 2 year olds if they never take the lead and jolly them along. (have visions of lots of 2 year olds with snotty noses, no dinner, etc etc cause they said no!)