Ah, I can do this one. I think.
This story is the classic old gag of dressing the hewro up as a
woman. Groo has to dress up as a woman in order to retrieve the
fabulous jewel held by a wimpy and lisping king. It's in issue 6 or
7, and the secret message reads something like "Hey, if you like this
comic then look out for Flashfire, written by Mark Evanier, released
soon by Marvel". The suggestion is made by the wife of the family
where Groo hides when he discovers he cannot get in via his normal
method of killing all the guards until he reaches the king.
Phew! PLEASE let me be allowed this since I really *really* know the
answer, look at all the detail!