I doubt that it will be made possible to write a reference to anyone. A solution for now could be to just regard the friend request as a consent feature. As in: Hi, I’d also want to leave you a reference and would be nice to receive one. You could then un-friend anytime after having exchanged the references. The other member will not be notified. Your friends list is also not public.
We already discussed the subject more generally in Profiles for couples and friend groups. I saw you had posted at the time that leaving two references worked fine for you on Couchsurfing. Though it seems most other members would prefer an option to link profiles. But maybe I’m biased because I’d also love to see the link feature and hope we can eventually implement it that way.