There is no help file, and I always forget to check this in game, but I seem to recall that years and years ago an ASSIST <target> command was added, with which you could start assisting <target> with whatever they were fighting... am I just imagining this, or does the command actually exist?

Yes it exists. You have autoassist on by default, but sometimes autoassist doesn't seem to work and you can also have it off instead opting to manually assist (or not) at your leisure.

Autoassist will work while blind. Manual assist will not.