Toyota recommends in both an Offensive and Defensive fire response that the HV battery pack should be allowed to burn itself out. Even with copious amounts of water there is not a high enough chance to cool the adjacent NiMH modules. The casing is designed to support the explosive ignition (once the battery module reaches ignition temperature) and allow it to burn out. The only danger is the gas that could/would come through the rear cabin vent.

Additionally the cell plates are designed to absorb any breach of the NiMH electrolyte akaline fluid which would be very caustic to humans. Toyota openly states that it believes any crash 'catastrophic enough to breach both the metal and plastic battery pack modules would be a very rare occurance - including rear impact'.

Toyota officially recommends that the vehicle simply be immobilized and disabled and that no forced structural entries be made unless all airbag systems have been deployed. (techinfo.toyota.com)

However, if you're really feeling boy-scoutish; you could place a vinegar gel-pack solution over the battery pack that would neutralize the electrolytes - in theory the pack would breach if there were an impact hard enough to break the battery housing.

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"It's not easy being green..." - Kermit the Frog