Blast from the past! One of those bigcoupe threads was me and my old E24, 15 years ago. eek
In my bodging, I used the standard "nut behind the plate" to hold the inlet rocker bars in place, and experimented with machining grooves in the rocker bars, to take a circlip (using the same circlip and washer as hold the rockers in place). But those locking tabs are a great design, wish I'd thought of that!
Is the thickness of the head bolt washers important? From the photos I think the locking tabs are not as thick as the washer would have been.
I fixed the issue on my E24 with a replacement head, from a breaking M90 engined 6er. The rockers in the new head were a much tighter fit and did not slide around.