Is this a valid way of "zeroing" the BUIS at home:

Set up the upper receiver + barrel in a Bushy receiver block on a bench vise. Mount a scope that has been zero'd already, find a target - mark it. Switch to the CH or BUIS and adjust front post to match what I aimed at with the scope.

of course I'd fine tune it at the range, but without any fancy laser boresighters or boresighters, is this "OK" ?

That works - just make sure that the scope was zeroed ON THAT RIFLE - if it was zeroed on a different weapon or if you changed or removed the barrel then the zero would be off.

A more common meathod is to look through the bore to determine where the barrel is pointing then adjust the sights.  It's easy to do if you remove the lower and the bolt carrier group then put the upper in a vice (or Workmate work bench).  IIRC this procedure is covered in the -23&P which is available for download from the Information button above.