It's hard for me to believe that there would be a rule against word of mouth but you're also responsible for anything you do offsite.
I think what you might mean is should you just tap their shoulder while behind them and initiate conversation with, have you tried this website.
If you're already having a bit of chit chat personally I hardly see the offence with common discussions like how did you find out about this place or have you tried ukpunting if that fits naturally into the discourse.
Unwritten rules are that punting is private/discrete and you usually leave people be but it can also depend on circumstances.
If there's a bleeding waiting room could leave a calling card or a brochure with the site on it but WGs might also pick it up and start looking at their reviews
.