Which plugin are you using to dictate whether a given zone is a PvP zone?
If using the DynamicPVP plugin from uMod, it would be quite simple to make an addon plugin which would communicate with both plugins to facilitate this. I have created a similar addon plugin for Restore Upon Death (https://github.com/WheteThunger/NoRestoreInDynamicPVPZone). The same concept could easily be applied to make an addon plugin for Backpacks, either by using the CanDropBackpack hook to block dropping if the player is not in a PvP zone (while Backpacks is configured to drop all the time), or by explicitly dropping via the Backpacks API when the player is not in a PvP zone (while Backpacks is not configured to drop).