Further investigation has bared a bit of info, there are schema files for StatTypes and ModifierEffectTypes but no xml files linked to these schema. I guess modding these is locked away for now, or forever.
At present, it's more or less the case that if something cannot be done with the things defined in the schema files that come with the game, you cannot do it. That being said, there are things defined in the schema files that are currently unused - for example, a fourth (and more importantly, functional but unused) type of fighter.
All this has lead me to the option of just replacing an existing fighter type to do what I wish. Which again is not ideal.
As I wrote above, most likely while you were typing this, create an EscortFighterBlueprint, define at least one class which references the EscortFighterBlueprint, and have the InitialEscortFighter lines in *ShipSetStyleDefs reference your new Escort Fighter class(es). There is nothing in the unmodded game which makes use of Escort Fighters; even if there were, it'd be redundant because at present the InitialEscortFighter for every faction is the same as the faction's InitialAssaultFighter, so even if there is an instance where Escort Fighters are actually used in the unmodded game (and I'm almost positive that nothing uses them anymore) you won't actually be losing any diversity. Once you've defined your Escort Fighters, you can add them to a ship component or starbase module using EscortFightersCap.
