Oh, another thing :

If all transmitters jitter over 'a few centimeters', how does that
affect the beam accuracy ? The jitter will create phase noise, which
will certainly affect the 'thick-ness' of the beam. Do you know how much ?

Rob

If the technical difficulties are overcome, would it essentially be

possible

that anyone
who would like to join in beaconing can buy and connect a small
transmitter/antenna in his/her/it back-yard ?

Actually, a next generation or so cell phone would be about right.
If it contained an improved GPS such as Galileo, the location could be

known

to a few cm.  If then the phase was adjuseted properly, a large
array of cell phones could make a very effective transmitter.