Does anyone make a 100A, 125A, 150A, or 200A service panel that uses
breakers for the circuits but has fusible pull-outs for the mains? I've
been looking at Square-D's catalog, but I don't see anything like that.

I don't need one; I'm just curious. It would be great for being
absolutely certain that you were not still connected to the grid if you
needed to backfeed with a generator during an extended power outage.
You wouldn't need a transfer switch; pulling out the main fuses would be
a positive mechanical disconnect.

Bob