And yes, at least, Winchester uses a metal plate of sorts in the base of their sabots. It's possible that their are others but I haven't really looked that thoroughly beyond knowing they are all expensive.
By the BLS seating thing, I know that a common complaint is the blown bases from the bullets cutting through. What I mean by my possible explanation is that it's more common than not to have heavy recoiling calibers crimped (I would say a 12 slug round would qualify) and in tubular weapons the ammunition is usually crimped as well to prevent the battering of bullets into the cartridges from the hammering effect like in most lever action rifles. Every single sabot type that I have cut open (out of curiosity) has a physical lip, or some sort of shape to it/them making bullet movement within the sabot impossible, short of discharge and exiting the barrel of course.
Obviously the BLS and RSS sabots can not have a lip because they (BPI) has no clue as to what 50 caliber bullet(s) you would be using. Different types of bullets have different ogives and shapes to them.
It would seem to me that after a few firings of these rounds the bullets, in the other shells, could quite easily slide under the recoil within the tubular magazine. They do in metallic rounds, so why not slug rounds?
If there is a separation between the base of the bullet and the sabot the forward movement of the sabot against the stationary bullet (even if for a millisecond) could quite easily destroy the plastic sabot base under 10,000 or more PSI. It would seem to me that could be a regular event if the bullets aren't seated within the sabots correctly.