Well, I think most hotel housekeepers would rather err on the side of caution when it comes to picking up money in a room. If they're not sure it's a tip for them, they're not going to touch it.
Remember as well, that many WDW housekeepers speak very limited English, and they may not be sure what "mousekeeping" means, especially if they're new to Disney. Or they may not be sure that fancily-folded money - or money in decorated envelopes - is for them.
OP, I know you want to add a little fun to the housekeepers' day by doing a treasure hunt, but I don't think that's going to work out the way you hoped. Housekeepers are on the clock. They need to get the rooms turned around as quickly as possible, as guests are (impatiently) waiting to get into them. Just as a restaurant server wouldn't be able to take time out from waiting on tables to do a treasure hunt for her tip, a housekeeper just does not have the time to be searching a room for their tip. Rather than making it more likely that they'll pick up your tip, I think it will pretty much ensure that they won't.
In the end, it's up to the housekeeper whether or not to accept your tip. I think if you've made it clear that it's for them, then you've done all you can. I usually just fold a slip of paper around the money and write "For the housekeeper - thank you!" on it.