Hello
I'll wave since I couldn't find the welcome smilie.
Now to the matter of your question
We have not been at MK for an after park closing reservation but we have done this at the Animal Kingdom, had a reservation about 10 minutes before the park closed. We did as you proposed, go to the restaurant to check in and were seated shortly. Once we were finished, it was rather cool to get to walk out of the park with nobody around. ANother time we had dinner in EPCOT at Rose & Crown, after Illuminations, we ponied up to the pub bar to have "one for the road" and ended up staying a little longer than we realized. Had nothing what so ever to do with that English beer
Walked out of EPCOT rather empty, yet still Cast Members were around to "direct" us out and evidently a bus had been disbatched as it was waiting on us and many others that may have been dining past park closing.
If relying on WDW transporation, never fear, they will get you back to your home resort
You do not say when you are visiting, but if MK is closing early for a party, (Halloween or Christmas) I'm afraid I can't say for sure how this may or may not change anything other than if you did not buy a ticket for the party, they will most likely be insisting you leave immediately after your meal. No passing go, no stopping in the shops, no hanging around to watch a special parade or anything else going on for the special event party since guests pay extra for these events.
If MK closed early for an extra magic hours nights for resort guests, if you are a resort guest you can stay to ride attractions (showing your resort room key to verify you are eligible) or you can meandor out, stopping in shops along the way.
In all cases, you will be able to get a ride backi to your WDW resort. ENJOY!