Not ideal, but my workaround (for the member site I'm launching in a few days) is to create:
one Member Area that's set to free access, and
a second Member Area that's a monthly recurring fee
I'm only going to toggle the free one on from time to time (it will contain sample content) as a promotion for new sign-ups. When I toggle it off, free users who have signed up will only see the per-month Member Area.
I would prefer to have coupon codes (or gift cards/codes) that grant temporary access. I imagine it's already on the wish list — right Squarespace pals?