The analysis of FOOM as a self-amplifying process would seem to indicate that in principle one could get it started from a relatively modest starting point -- perhaps just a few bytes of the right code could begin the process. Or could it? I wonder whether any other considerations give tighter lower-bounds.

Something like a primitive bacterium ignited the current living explosion. However, that took billions of years for the explosion to grow to the current level. We don't want to recreate that. What we want is to have a system that builds on the developments that have taken place so far. That means a man-machine symbiosis. Going back to square one with a machine is not a realistic possibility - so the size of the smallest pure-machine system seems kind-of irrelevant. Big enough for it not to happen that way. Pure-machine systems just get their lunch eaten by the man-machine symbiosis at the moment. They can't compete with the combined strengths of hybrid systems. The way you get a pure-machine system at the end of it all is via automation within the existing symbiosis.

This is the same as with the problem of creating life. You don't do that by starting with a self-replicating machine. Rather you have a meme-gene symbiosis, to help get the new organisms off the ground, and then gradually automate.