I'd like to add a subpanel for a workshop and am wondering if it
absolutely has to be wired off of a breaker in the main box or if it
can be wired from the meter (i.e., can I have two main panels).

You would have to check your local codes, but we have three separate
service entrances (main panels).

Our electrical meter is located on a pole out at the road. Then the power
comes down to a junction box behind our pump house. Separate lines then run
to individual service entrances at the pump house, garage, and our house.
Each building has it's own main breaker, grounding electrodes, etc.
Completely seperate and isolated from each other (other than the incoming
power).

We built the pump house first, then our garage, and finally the house. So
we wouldn't have been able to run a subpanel off the house anyway.

However, I did run a small 30A circuit off the house panel to a small
subpanel in our storage shed.

Anthony