s2d,
The regulation regarding a tailwheel endorsement says you have to train them in wheel landings, three point landings, and crosswind landings.
I submitted a query on this to our local FSDO, specifically relating to skis. They didn't have an answer, but said they'd query FAA Legal, which they did.
FAA Legals' response was that you could not meet the requirements of "wheel landings" on skis, therefore you could not legally endorse someone for a tailwheel endorsement if they had only flown a tailwheel airplane on skis.
And, no, I am not making this up.
In any case, look at it this way: Suppose you fly with someone on skis in a tailwheel aircraft, and endorse them for operation of a tailwheel aircraft. Now, spring rolls around, and that person takes their tailwheel endorsement out and hops in a Cessna 185 on wheels, which he or she is, according to you, qualified to fly. Have they done wheel landings? Have they done three points? Are you comfortable that you've actually given them instruction in a tailwheel (emphasis on the wheel part) airplane? I'm betting the FAA would come out to visit you.
There is no way that flying a ski equipped airplane in any way shape or form qualifies a person to fly that same airplane on wheels, in my opinion.
Furthermore, you cannot place conditions or provisions on a tailwheel endorsement, such as "no wheel operations". Either they are qualified in a tailWHEEL airplane, or not.
But, hey, ask the FAA your own self, and let us know what they say,
MTV