Of course when it comes to unrealistic personal ambitions, I wouldn't put anything past a public figure like him. But I highly doubt he's angling for a spot on a ticket, especially the Republican one. Mitt has to shore up his base with a strong conservative. Somebody who endorsed Obama in 2008 is pretty much the opposite of that. Powell's never struck me as a genius, but even he has to know that.
I suspect he's angling for attention and trying to restore his credibility after his disastrous turn as SOS.
And pimp his new book that I'm sure tries to make himself look good.