Nominal value of legal tender is in fact legally defensible.
Mind that the problem is actually the inverse of what you describe. It's not that the nominal value is greater than the intrinsic specie value which causes problems with coinage, it's where the monetary value is less than the commodity value of specie, in which case "bad money drives out good". I've already discussed that in detail.
One point worth making explicit is that the receiver of such an under-priced coin would be more than happy to receive it, it's the spender who has to weigh the loss in commodity value against the nominal transactional value, should their counterparty only agree to acknowledge the latter. This brings up the further point that in an exchange, transaction price (whether nominal or commodity) depends on the alternatives available to the parties. A spender without alternatives on price or obtaining desired goods/services might well spend a higher-commodity-valued coin at its nominal value. Should they be aware of that difference, they might well not be happy about the fact, but they'd be forced into the trade by circumstance.
Mind that the problem is actually the inverse of what you describe. It's not that the nominal value is greater than the intrinsic specie value which causes problems with coinage, it's where the monetary value is less than the commodity value of specie, in which case "bad money drives out good". I've already discussed that in detail.
One point worth making explicit is that the receiver of such an under-priced coin would be more than happy to receive it, it's the spender who has to weigh the loss in commodity value against the nominal transactional value, should their counterparty only agree to acknowledge the latter. This brings up the further point that in an exchange, transaction price (whether nominal or commodity) depends on the alternatives available to the parties. A spender without alternatives on price or obtaining desired goods/services might well spend a higher-commodity-valued coin at its nominal value. Should they be aware of that difference, they might well not be happy about the fact, but they'd be forced into the trade by circumstance.