This is the same issue I have reported in "Wrong answer" when courses have alternatives. - My Little Word Land
How about a / or even \ (the latter may never be used).
Otherwise just look around the keyboard for unused characters ;-)
Presumably in time we would go over the ex-MemRise courses we support and amend all the entries to the agreed separator. And this would fix the alternatives being accepted as a right answer.
Finally we could add to the Course description that it has been updated to work on MLWL.
(My request for tapping or selecting words instead of typing still stands. See Tapping tests or block answers - My Little Word Land)
BTW - thank you for coming to the rescue of 'our' MemRise courses and the amazing job you are doing on getting them mirroring the MemRise feel.