I did thankfully tell her this evening, and it went great!
I will try your idea though! thanks Em Es!
Well if you did and since it went well, this advice is outdated, but it may still come handy if you sometime in the future in a similar situation.
Is not as much as being honest, as much as seduce your counterpart into "it", whatever "it" is. To seduce is in essential to sell something to somebody. Seduction is to induce a behavior in another person without this person realizing that he/she is driven towards such behavior. Like some abyssal fishes that "lure" their prey into their mouth cleverly camouflaged as an inoffensive cave.
You want to be tickled to death but this other person, then you will try to induce the behavior in that person. You want to "sell" your wish and desire to your friend. Very often like selling a house or a car, is not enough to just say"Hey this house is wonderful pay xxx for it", "I want to be tickled, just get into it". You need to show the client the advantages of your product and how is the best choice. You need to "seduce" your friend into tickle you to death as if it was her choice not yours, or that is happened by chance, albeit you were waiting for it like the spider waits for the fly.
As how to do it, well, there is no sure recipe, but when I was a teen, and wanted to tickle a special girl in the long summer days, I went into record "North to Alaska" which has a great tickle scene with Capucine, and put the machine just to that five minutes before that point on the tape, (videos...ancient history!!), to have time to explain her the plot before the scene. When she arrived I told her I was watching a movie, and put it on. My hope was to get into the topic of tickling without looking like salivating for tickling her. It did work, the scene is intense, so the topic was on the table, and I was not guilty for bringing it up, it just happened. And from there to talk about where are you ticklish?, does a feather tickles?, is tickling good to play true or dare, etc. and so on until a tickle fight break up it was rather easy. She never acknowledge that she liked to be tickled, but later on that summer, she would do silly things like to spill my coke onto the floor without apparent motive and say "I do what I want, what are you going to do? Tickle me to death? I dare you to try wimp". So she liked it albeit never acknowledge it, and I did not pushed. The next year her parents move away so we never talk again about our tickling fights.
Today you can use Wikipedia, or google, and play the game of finding "odd" things into the web, in order to bring tickling to the conversation. In your case you can "stumble" into the TMF, by chance. "By chance" is the key point not to look as dying for it. The game of write a word in google and see what come up is excellent for that. Nothing wrong with straight honesty, but having a excuse to talk about it is the best to taste the waters and open the mind of the other person, before he/she rise his/her shields and labels you as a weirdo.
Sorry to write so much, but your "problem" reminded me all the effort with the movie North to Alaska to find a excuse to tickle that girl without knowing what was going to be her reaction.