my apology if someone had already mentioned this.

Found - English Armoured Sergeants - can be recruitable (kind of). If you get a decent diplomat and work on the rebel/other fractions stacks with armoured sergeants, you can get the armoured sergeants.

+ armoured sergeants are upgradeable, unlike the mercenery spearsmen and
are cheaper to maintain. (same as other fraction armoured spearmen)

- I dont think armoured sergeants can be retrain (not tested)

Note - this is vanilla 1.0 MTW2

further thoughts - I wonder if there are other "hidden" units that can be bribe.