Has anyone ever seen or heard/read about an arming doublet rivited with plates (like a brigandine) on the back only?
I have a 15th century gothic style armour but no backplate. My arming doublet is quite thin with chain voiders and a skirt... But I have no padding whatsoever on my back. Ouch. I have just ordered new shoulder armour from Skt. George armory (fitzherbert effigy style) which somewhat protects my upper back... but still, OUCH!
The easy but expensive way out is of course just to buy a backplate... but my frontplate is not designed with a backplate... what to do.

Until well into the 15th century, breastplates or pairs of plates were normally worn over a haubergeon (one of the sources on Agincourt remarks that many of the French wore hauberks and mail leggings under their plate, which suggests that this was no longer usual at the time of writing). This continued well into the 15th century; you can see the sleeves dangling over the armharness and under the shoulder defenses on some late Italian armour, I think that any white harness without a complete breastplate, backplate, and belly/thigh defense was designed to wear with a hauberk or haubergeon.

Iv'e seen illustrations of exposed plackards (lower half of a breast plate) with brigandine tops and backs. They seem to be connected as one piece of combinaiton armour rather than the plackard simply being worn over a brig but I couldn't really tell just from art. 15th century if I recall correctly.
Does that help?