Enhancement? only alow membership type once, or does this exist already somewhere?

If there is a way to currently do this let me know.
I needed a method to allow individuals in groups we are marketing the site to, allow them to get an idea of what the site can offer their users as a free full function trial feature for just that VIP member. We did not want users to have to enter paypal info to get this free demo so the coupon codes were out, along with the fact that you can not charge 0.00 to paypal and the fact that you can not limit the number of months a free promotion is applied to.
My way around this was to set up a password protected promo page where we could give the link to one of these VIP trial users which has a membership plan that does not charge and lasts for 2 months.
What I want to do is limit the user to only signing up to this plan one time and then never use it again, I do not see a mechanism to do this (such as only allow this membership one time per user)
If this is not possible yet I think it would be a good enhancement to add to memberships where a flag is set for a user once they select that membership, and when it expires, if they try to use it again the membership 2 plugin see's that flag and notifies them that they must select a different type of membership, the promo was a one time selection.
Can this be done now? and if not would you consider it for an enhancement? I think it would be super easy to implement.
Thank you again, your support has been excellent.