You'd be able to use an old Mac as a bridge. For software, Laserwriter Bridge will work, as will Localtalk Bridge, as will Apple Internet Router, as will an Appleshare Print Server (with LW Bridge also installed, or even better, and more interestingly, Apple Internet Router - you get zones that way). But for all these you'll need another Mac with both a localtalk port and ethernet in order to network your LWPro. OSX continues to use Appletalk for print services, so the printer will be perfectly functional with any such setup.
I have a localtalk only LW4/600 connected to an Appleshare 4 Print Server/ Apple Internet Router machine (LC475, System 7.1). This setup is rock solid (as in, it will run for months without rebooting) and it works flawlessly with X.4. I don't have X.5 so cannot speak to it.
Of course, you could always upgrade your LWPro 600 by buying a LWPro 630 logic board. Then you'd have the aaui ethernet port that you wanted in the first place.