I also have an older HP Laserjet, a 6MP, that I wanted to keep is its print quality is superior.
I used this adapter cable usb to serial and it didn't work.
Then I bought a print server, an HP Jet Direct 300X, connected the printer to the Jet Direct, and the Jet Direct to the ethernet port.
Works great. (downside: more cables, and I must say I forgot how I configured it. But it is doable. Apple talks has to be on. Airport (if you are using it for the web), HAS TO BE OFF FOR PRINTING, OR THE PRINT INFO GETS LOST SOMEHOW. (once I forgot it, and it worked, but usually it doesn't, if airport is on).
The JetDirect once was pretty expensive, but now it's outdated, and can be had for 25 to 30$ on eCrook... ahem, eBay.
Outdated stuff, that is cheap, but still works, is pretty much the only product one can buy on eCrook... ahem, eBay.
I also remembered that I had to buy a crossover ethernet cable connecting the computer to the server.