The thing is, there is such thing as primitive battle armor; they are called exoskeletons, and have been around since the early spaceflight time frame. Years ago I speculated on these boards what commercial armor for exoskeletons would be like (
here)
I love your concept of retrotech BA, but Prototype Standard Armor is not, in fact, a low-tech version of Standard Armor, but an unoptimized one. If you look at the "Battle Armor Structural Costs and Availability" table (pg. TM281), you'll notice Standard and Prototype Standard are both Tech Rating E and cost the same amount of cash; therefore, if you have a factory capable of putting out Prototype Standard Armor and bother to do any R&D (or industrial espionage) at all, you'll eventually be able to build Standard.
As a matter of fact, building BA chassis already requires a Tech Rating of E, so it's pretty advanced for the Clan Invasion era Periphery powers. Exoskeletons and PA(L) are respectively TR C and D, so it should be easier to build and maintain even in relatively backwater planets.