Question: I am just curious if last minute hotel deals is actually a thing. I am not a big traveler and have no clue.

But in theory if a hotel has a bunch of empty rooms for the current day, it would make sense to rent them at a reduced rate as opposed to letting them be empty.

Is this actually a thing that could be leveraged?

I am going on a road trip, and the last leg of the trip could take many variations on a moments notice and was wondering if this is something I could take advantage of.

Answer: 
After seeing the comments here, I tried it myself.

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Can you get it? - Yes you can, still today.
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I asked in China Eastern and Korean Air, both in economy and large cabin, in 2016.

You can ask an attendant once you finish eating, and he/she kindly starts to check if they can serve it for you.

However, whether it is only a main dish or whole dish depends on the airline (and attendant oneself and also random chance). In China Eastern I only got a main dish, but I didn't like to bother to complaining about it (and it was enough), so didn't make any further actions.

When you ask, the tips by other answers would likely work to your favor; be polite, wait for the end of the distribution of meal, and don't disturb. However, I want to add one more tips; you shouldn't ask it shortly before the start of collections of finished meal. In this case, attendants have to care about *only you* once after they finish collecting all the other passangers' meal. So make sure your second meal is going to be collected with others'.

However, some airlines don't allow it for whatever reasons. I asked at Asiana Airlines a few times (different flight, route, and crew) but I got declined in all of them. In fact when I asked one attendant she immediately got to ask a more elderly attendant (maybe a chief) and the answer was NO. So in these airlines, you could never get an extra meal, unfortunately...
I recall once that a special-order meal got mixed up and there weren't enough of them (vegetarian), so the cabin crew scrounged around and came up with a couple meals' worth of snacks and the safe sides of extra meals for that person, instead... and with the offer of more, if they were still hungry after. 

Obviously this was something of a special circumstance, but the point is that those snacks would still have been on the plane, if the original meal hadn't been mixed up. If someone asked politely, there's a pretty good chance those extra snacks, or the other portions of the extra meals, could still have gone to someone else if they were still hungry even on that flight.

The cabin crew usually do their best to accommodate requests like that if you ask politely. They may be willing to put in a little more effort, like borrowing snacks from a higher class, if they know there is a reason for the request - something like a person being hungrier because they couldn't eat before the plane for whatever reason, or needing a larger meal to balance their blood sugar or to replace part of the meal they cannot eat. It might matter how long the plane ride is (and thus, how long until they know you can find something else), or probably a whole host of reasons based on how someone looks or asks, and how urgent or casual the crew thinks the request is.