Thank you for testing this. It's very puzzling.
I get exactly the same results on two different Mountain Lion systems. One of them is on a dual-boot MacBook; on that dual-boot system, under Lion I can print to a shared LPD printer very easily. The exact same SheepShaver system that prints under Lion can't print to a shared printer under Mountain Lion - although it can print perfectly easily from SheepShaver directly to an IP address (a networked printer) on the network.
"The printer cannot be verified" is the message I get every time when I try (in SheepShaver) to verify a shared printer in Mountain Lion. The printer is definitely shared in Mountain Lion; the firewall is off; and stealth mode is NOT on if I temporarily turn on the firewall to check whether stealth mode was previously selected.
Here is my cupsctl output in Mountain Lion:
BrowseLocalProtocols=cups dnssd lpd
This looks right to me. If anyone has any insight into this, I'll be very grateful.