In fact it is now routine to build GRP one-off boats in a female mold made specifically for that one-off. It is done with large 3D gantry routers, machining a mold from hard foam. That is a little more expensive (but not a lot) than building a plug or setting up a bunch of station molds. One advantage being that the weight and labor of fairing is greatly reduced, which partially or wholly recovers the additional cost.

My boat was built over a planked plug that was then thrown in the dumpster. In either case (female or male mold) "The rest of the design remains fluid, and is open to change right through the process", whether built from GRP or wood. The constraints you speak of have to do with tooling, that is a time saving convenience for series production. Nothing in my all GRP boat was tooled, save for one-off tooling created when necessary (and which would also be necessary if built in wood). Many of the details would not have been practically buildable in wood.