I think it won't work as a simple serial connections between these two boxes - which IMO is stupid - it should. I looked through the tarballs of netatalk version 1 and 2 and there is no mention anywhere or LocalTalk or serial communication. I don't know why, I just don't think they ever implemented it. Maybe somebody can fork netatalk 2.2.5 and add serial to it. It's beyond my abilities.

What you will probably need to use netatalk with a LocalTalk only printer is a LocalTalk bridge. This is either a dedicated hardware or a Mac which has both ethernet and serial running a program which translates the messages and communicates them through the serial port.

I know it's not what you want to hear! LocalTalk seems to be its own protocol, and Mac serial is RS422, not the RS232 of most servers. Adapting the different types of serial is not too hard, but for whatever reason the protocol end of things seems to be black magic that nobody knows much about.