Yes, I think the only way a DLR hotel would allow that is if you were part of a convention, I know my fil asked once when were staying there if he could get some sort of half day ticket and he was told no. Now that was about three years ago so it might have changed. You could call the DLH directly to ask them.
If you are doing park hoppers, it really isn't that much more to add a day on them, even if its for a partial day. I have been known to purchase them for five days, even though one of those days I may go in for an hour, close to closing. Its about $10 well spent. If you only have one day total, and its after 4:00, you still have to pay full price. And me being me, I'd probably still do it just to go for a few hours!