I was thinking something like "tongue-in-cheekly" but it sounds awkward. Of course, alternatives are welcome, but I couldn't come up with one that conveyed the half-serious playfulness that I attribute to "tongue-in-cheek". Any thoughts?
(Edit: I reworked my answer, since Merriam-Webster was inconsistent between the free and unabridged versions.)
Per OxfordDictionaries.com, the adverb and adjective form of the phrase "tongue in cheek" is tongue-in-cheek (with hyphens):
[as adjective]: her delightful tongue-in-cheek humour
[as adverb]: I think he was talking tongue-in-cheek.
I'd say tichily, though this does rather depend on people being familiar with tichy as an "acronym" for tongue-in-cheek.