If this person ever had urinary issues,he might now have some knowledge of the subject of incontinence. Your excuse for wearing diapers is lame at best. With your current plan of attack, I guarantee he will......rightly or wrongly, regardless......get the impression of one of three things;
1/ You are looking for a sexual relationship with him.
2/ You are a troll, if not why do you have to remind him about his past incontinence issues.
3/ Along with number two above, he firmly believes you really hate him.
Always being one for interesting times, that is one road trip I'd give my left arm and right hip to be on......hell, I'd even sell my soul to the devil.
My genuine advise, go back to the drawing board, but only do so when you've worked out what you really want from your friend and from this trip.
PS: Please keep us up to speed on this, I don't want to go to heaven before I hear how this all pans out. The suspense is killing me already.