I'm assuming that it is just the setup/status/diagnostics control panel so you'll not be able to print anything arbitrary (shame, it could be a fun game!). If you are of a mind to wind someone up you might be able to kick out a pile of test pages and reconfigure the thing so it is no use until someone does a factory reset.
A similar but worse case was some years ago when a range of consumer router+firewall boxes had a fault which made them present their control interface on the WAN interface and had no password set by default. A large number of those somehow got into a search index (it may have also been Google, I can't rightly remember), and from there you can probably do more harm than you can from a printer.
I've looked at two and both had the option to print a file that you uploaded. Of course I didn't actually try to print anything, but it looks like you probably could.
I may or may not have just printed out some random messages for people to find.
There is something strangely compelling about sending thoughts out into the ether with no chance of feedback. Fax pranks are before my time, but I totally get it. I hope I made somebody smile today.
The other question, which would be fascinating to see raised in court I feel, is whether a printer is a "computer" within the terms of the law (CFAA, CMA(UK) or whatever).
You'd probably be able to question the meaning of access too - for example if you find an IP on Google and simply send data to port 9100 that's not really access, accessing a computer is 2 ways. If the law judged spamming port 91 as "access" then sending faxes or texting someone would come under the such legal acts .. that can't be within the intent of the law surely.
If other laws are used - "you sent them a message they didn't want" - then that's the end of [legal] unsolicited mail [yay!].
A similar but worse case was some years ago when a range of consumer router+firewall boxes had a fault which made them present their control interface on the WAN interface and had no password set by default. A large number of those somehow got into a search index (it may have also been Google, I can't rightly remember), and from there you can probably do more harm than you can from a printer.