In the HV20 manual, page 44 it reads "You can verify the current version of the camcorder’s firmware. This menu option is usually grayed out."

But what if it's not, and does that mean it's updateable, maybe on the next version?

It's grayed out only if it's never been updated, such as in a brand new camcorder.

Assuming that there is ever a firmware revision, yes you'll be able to update it.