I just got to thinking...
What about using a cart machine designed for radio? They sound great, are essentially a programmed string of samples that execute by a single button, and you can plug in a whole set or a single song. Really, in my mind this is probably the best option. The 8-track type carts are a little expensive but they can hold a bunch of shit. I bet you could buy out an old radio station's carts and machines during a conversion to digital automation.
If you would like to hear an example of what they can do, listen to any early Negativland lp. They used cart machines in leu of samplers. They still use cart machines live as I understand.
The answer is the radio cart machine. Foot switch to advance the carts is a really easy mod.
It's probably EXACTLY what yer looking for. No limit on 'sample' length. There might be a problem 'advancing' an early deck, and if you wanted the 'advance' feature yer talking about adding another foot switch (one for play, one for advance). Now if you wanted to get tricky you can add a record button, but these decks are pretty tricky to record on the fly with, so that's something I wouldn't suggest trying. These decks can get real tight when you get the hang of them - you can fire off a bunch of smaller 'stingers' in a row (with a somewhat 'random' effect) and you don't have to worry about stacking them into mush.
When I worked at old cart stations, which are still rather 'popular', these carts were the backbone of live broadcasting, so much so that they influenced how radio was done.
Fun machines.