Mega Man is a cameo character in HTFA 6. He only appears in the end of the credits roll after the Dunami logo is shown. There, he is seen holding up a sign written in a hilariously broken English, persuading fans to not leave the HTF fandom.
Mega Man is added in the updated version of HTFA 6 (which switches some of the characters' voice actors to the new ones). The initial release didn't have such cameo.