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: 
There are "extra" meals so you can certainly ask.

If you are say, a first class passenger, the extra meal will be given to you almost as a matter of course if requested. You have "first dibs" on this (and other) privileges.

If you are "coach" class, your request will be granted more often than not. A problem might arise if first class, or other coach passengers asked ahead of you, and they're "out." This might be particularly true of a certain meal, as opposed to "meals" generally.

The alternative to giving away an extra meal is that it is thrown away. Most attendants would rather give the meal to someone than to discard it. You will not be charged.

Note: This answer does not apply to "food for purchase" situations. Then you do have to pay for each meal that you eat.
Airlines: Emirates
Route: Dubai to India (CCU)
Well, I got encouraged by the answers here and asked.
The answer was a "No".

In the airlines defense, I asked for a Hindu meal, which maybe was limited in number.
But, guess what my co-passenger told me.
If you had asked it in your flight from Dubai to some western country they would be more than willing to serve you.

But here you are going back to the third world. You are lucky the "No" was not a rude one.
Ouch... :(