This depends on definition of "simple".

You might for example consider the following code:

   mov   dptr,#SetbAddr     ;address in "vonNeummann" external RAM
   mov   a,#0D2h      ;opcode for setb
   movx  @dptr,a
   inc   dptr
   mov   a,r0         ;our bit address
   movx  @dptr,a
   inc   dptr
   mov   a,#22        ;opcode for ret
   movx  @dptr,a
   lcall SetbAddr

... but it requires the presence of external RAM wired up for both code and data address. It can be stripped down upon subsequent use to bare

   mov   dptr,#SetbAddr+1
   mov   a,r0
   movx  @dptr,a
   lcall SetbAddr

if the RAM content of surrounding addresses remains intact.

Is this SIMPLE enough? :-)

JW


PS. This method has the advantage over the "ROM-only" method in that the WHOLE bit addressable space will be served, including the bit-addressable SFR area - this is quite cumbersome using ROM only.