external hard drive via Airport extreme for libraries, possible?

The Aperture library needs to be on a volume formatted MacOS X Extended (Journaled). SO if you use such a volume with AirPort it will work in way, but this setup will be slower than a directly connected external drive using a fast connection, and it is not recommended by Apple. You'll be much better of with a directly connected external drive using thunderbolt or USB3.

See Apple's recommendation: Use locally mounted Mac OS X Extended volumes for your Aperture library

If you're asking if you can put the library there then the answer is no. Performance will be bad and the risk of corruption to the library is high.

If you want to put the referenced originals there that is doable.

I am guessing the best solution then is to move the whole library to an external disk if I want get MBP disk space back.

Right.

Make sure the drive is formatted Mac OS X Extended (Journaled) and then drag and drop the library to the drive.

If you previously force quit Aperture or it crashed, and there is a slight possibility of library corruption, I'd repair the library before copying it to the drive.

Test the copied library thoroughly, and update your backup, before deleting the original from the MBP.

I was thinking of keeping the references on my MBP

This is backwards. You want the library on the MBP drive and the originals referenced on an external drive, either local or NAS.

I am guessing the best solution then is to move the whole library to an external disk if I want get MBP disk space back.

The original files take up the most room in a library, moving those out and keeping the library on the internal drive will give you the best combination of space reduction and speed.