but I don't need a dirty trick to use the Keil C mechanism. I just need the information about how to use overlays in (complete) assembler with the Raisonance IDE.
I would suggest you try it with Raisonance. In many cases the compiler/assembler/linker makers do not think much about the pure assembly cases. While that may be sad, you can not blame them for putting the main effort to where the most customers are.

I just happen to know that OVERLAYABLE in assembler modules called by C do work (at leat in Keil).

Sometimes you do need a dirty trick to overcome shortcomings in the tools.

Erik