I think it gets murky quickly if you pay their dues as the employer- if you're funding the union there's a pretty clear conflict of interest. To me the clearest way to address this would be to offer a stipend without earmarking it so they can fund the union (or not) at their own discretion.

Another option would be to just formalize it as an ESOP, thereby erasing the distinction between employee and employer and effectively obviating the need for a union in the first place.