Can I add a 'service entrance barrier' to a QOC electrical panel?
PKSB1HA will definitely not fit a QOC 40MW225 panel, and probably not any QOC series panel.
They have an FAQ which says:

Does Schneider Electric offer repair parts for the Old(25+years) QO
Load centers? (For example old QO--MG200_ _ , QO--MW200_ _ or
QOBW3150-1)
No, the older 25+ years or so QO load centers are all obsolete, with no repair parts available. Consider replacing the complete load center
with a current design

That was discouraging but still a bit vague, since I wasn't looking for &quot;repair parts&quot;.  I emailed SD to ask about the barrier specifically and they replied basically just citing the same FAQ. I decided to just try the PKSB1HA and see.
It absolutely does not fit. With the panel open (*) a visual inspection made this clear - the main issue is that the two lugs are much too close together so that the two legs of the barrier overlap considerably. There may be some other dimensions that also would prevent a fit even if that were not the case.
This still leaves open the very slight possibility that some other product could exist, but that seems to be very unlikely.

(*) I had taken precautions with the cover off including wearing 1000V rubber gloves with protective leather outers, and of course having the main breaker off to limit potential 'hot' contact points.