You are entirely correct. Full cups itself is way overkill if you use a networked printer (which is pretty much the only option for a device like the N900). Cups contains all the bits and pieces necessary to manage a directly connected printer, that's why it's so big.

That's basically what the good old bsd-lpr or lprng applications can do, so they're pretty small. I still have lprng running on my N800. It can be done by a self-made script too though, as you describe. What lprng gives you in addition is a local printing queue.