Unfortunately, similar to ticketing, when you select multiple assets to update at once it is only able to allow you to interact with the standard/baked-in fields relating to assets.
Anything further than that would require an enhancement to the system to allow additional fields to appear.
Feel free to submit that as a feature request though! https://solutions.teamdynamix.com/TDClient/1965/Portal/Requests/ServiceDet?ID=2148
Sr Support Consultant