Is it plausible to depict the Segmentata using armor upgrades?

[Mediolanicus]

Using the M2TW engine, you can actually depict the more advanced set of armour worn by your units. For example, a non upgraded unit may not wear any armour at all, however, if you bought some armour workshop and re-train those units, you can give them better armour.

This can be an accurate depiction as legionnaires do not wear that armour until the later part of the Roman empire, and that armour isn't that widespread.

So, by making the armour available when you have a high level armour workshop, you do not have so legionnaires wearing the LS, and you do have a chance to provide your troops with that armour until the late game period.

Even if you did manage to equip those units with the Lorica Segmentata, only a few legions can afford it.

This all very possible and true, but I don't see what it has to do with EB.
It is also possible to add musket units to all the factions. MTW2 engine has them, and the whole of Europe used them at some point in history. But I don't see what that has to do with EB either...