This is an excellent question and one that I can't (so far, I've got a call in to USAFA) get a "definitive" answer to.
The short version: YES, you can receive a commission after leaving a service academy.
"Once upon a time" there was either a law or "rule" or "reg" that stated that you could NOT be commissioned prior to "your class" being commissioned. Example, you resigned after two years in the class of 1983 (out in 1981) you could not go to ROTC, or OTS and be commissioned prior to the USAFA class of 1983 being commissioned. But that was the only restriction I ever knew about.
And I'm not sure that's in place now; as I said, I've asked the question. If I hear back....
Steve
USAFA ALO
USAFA '83