You are contradicting yourself in a way. If they are printed on demand, it's extremely unlikely they are actual spot colors/ pantones. More likely they are just generic textile inkjet colors and in such a case they are strictly based on RGB conversions. That and of course PNG can only ever be RGB in the first place, furthering this point. So whatever you do, using Pantone colors will not extend beyond the point of getting converted to regular RGB colors on export if this is indeed the scenario. That doesn't mean you can't use them, but they realyl have no relevance for the actual production process.
Mylenium