yes, i think better just do a standard vchar field and call it domain_registrar to fetch and store data.
but here is bit tricky. personally, i only use registrar vendor just for domain, rarely i have same vendor for domain, hosting and email, as these additional services, usually are provided by MSP itself.
also, if i have a client who doesnt have the domain + additional services on MSP end, or at lease have a reseller account on provider end, i wont do any relation as the plan would actually be to move the domain where MSP can easily manage it, but for that reason, would be handy to see where the domain seats at that moment if you get me.
I`ll be happy to have it implemented in best and logical way that will work for IT Flow anyway 😃