The wiring harness at the head unit is a standard '07+ nissan harness - meaning it only has front left and front right outputs (as well as the same rear set) - no connections available for crossed over outputs. However, there could be a passive crossover somewhere in the wiring that would be a problem - but only if it can't be found.
I've got my polks in now, they're nice and light. There's no reason they shouldn't fit in the front or rear doors physically, but I'll confirm as soon as my work in the garage is finished (probably another week or so). I'll see if I can track down crossovers as well. Having said that, a common factory configuration is a simple capacitor on the tweeter and open connection to a midrange that is simply not very responsive to high frequencies, so it may still be fine as long as the tweet is disconnected.
I'll be putting them in the rear, though; the front doors will only be getting the midbass from my PC3.65c component set (mids and highs will go in custom fiberglass pods I'm building and installing in the tiny front window areas).
As an additional note on the wiring harness - I plan to use an Alpine imprint factory integration unit, so I need speaker-level inputs - but I don't want to cut any wires. So, I got two wiring harnesses - one factory replacement style, the other the standard type for connecting an aftermarket head unit, then chained them together to make a short plug that connects between the harness and head unit. Now I can cut *those* wires all I want without any impact to the factory original wires. The only problem I had was that the aftermarket unit style didn't have 4 of the wires (related to steering wheel controls and other things that wouldn't relate to an aftermarket unit normally), so I got two of the aftermarket harnesses, then used a small pin to pop the harnness pins out of the second one. I then used those wires I pulled from the secondary to fill in the missing pins in the real harness.
The part numbers are metra 70-7552 (connects to factory harness, need two of these to get the extra pins) and 71-7552 (to connect to the radio).