My understanding from some conversations I've had is that that EBRT as first-line treatment is always given as the max dose allowed, so you can't have it again because you already had the max dose.
However, there are some things that can mitigate this. If you had EBRT some time ago, such as before IMRT, the max dose was lower, so you may still have some spare headroom by today's standards. Also, if the mets are well defined, modern highly directed beams can be used to target them, although that might mean private treatment.