Xconq version 5 had a sometimes-useful flag called "setproduct" that could be set to false, with the effect that any attempts to change construction were disabled. So for instance, a city that was set by a scenario to build bombers would then build bombers throughout the game. The advantages were both in realism (retooling a factory can be very time-consuming) and in playability (no construction planning required).
To emulate this in version 7, you can set acp-to-toolup
to be zero for cities, but at the same time require 1 tp for each
type that the city can construct. In the scenario, set the value
of the city's tooling to be 1 for the one or more types that you
want it to specialize in (maybe switching between fighters and
bombers should be possible, but not to submarines).
Players can then start and stop construction as desired,
but are limited to only particular types.
Even captured independent cities can be limited in what they
can be used to construct.