Here's some suggestions, of varying power level:
The wearer of the armor can Meld Into Stone 1/day.
The wearer can absorb the armor into his body, gaining its armor bonus as a natural armor bonus (and wear more armor on top of it!), but takes on a grisly, were-ratty appearance when doing so.
The wearer can Hide In Plain Sight in underground locales.
The wearer is protected from involuntary form changes (petrification, disintegration, polymorphing, involuntary assumption of the were-form).
The wearer attracts rats and can speak to them (Summon Swarm and Speak With Animals at will, but only rats).
The wearer can become incorporeal as an immediate action for 1 round, 3/day.
The wearer gains a bite attack (1d4 + Str) with a disease effect (Filth Fever) by speaking a command word. He also grows a prehensile tail, which can be used to hold items, but not to attack, and which grants a +4 bonus to balance checks.
The wearer is at risk of contracting Lycanthropy whenever he takes damage while wearing the armor (Fort save each time...). Not so much a boon, but might still be considered cool!