Is there any way to run binary code from RAM?
In my opinion the interpreter idea is much more feasible. The notion of writing, in assembler, code that itself generates machine code, in RAM, and then executes it, is itself incredibly complex.

You haven't said why you want to do this, so this is somewhat of an X-Y problem.


  I don't want an o.s. in the way ...


The normal Arduinos do not have an OS in the way - basically the code you write is what gets loaded onto the chip.