Nick from Castle Dragonscar pointed out that issue 49 of Dragon included a full-page ad from the Dungeon Hobby Shop done in the same style as those mailing envelopes, but this time with speech baloons.
The title of the comic is called "Jeff & Ernie presents ... Dungeon Hobby Shop." Ernie is clearly Ernie Gygax, who managed the shop, but I'm not certain who Jeff is, though there are several possibilities who come to mind. Anyone know for certain? I also discovered that the artist "L.R." is Laura Roslof, wife of TSR artist Jim Roslof.