This is ameliorated by having more than one beacon that publish simultaneously, and xoring them all together, but a Bad beacon could just delay until it has see

This is ameliorated by having more than one beacon that publish simultaneously, and xoring them all together, but a Bad beacon could just delay until it has seen all the Good results and then spin its RNG until the full XOR gives what it wants... If you have very precise synchronization you could conceivably use the lightspeed limit to guarantee any Bad Beacon(s) can't know the output of the Good beacons before it publishes its own value.