A workaround is certainly possible, but this one's resource-intensive. It only uses one mïsn resource and one shïp resource, but it also uses up one precious active mission (out of a max of 16 concurrent missions).
Make a ship that is invisible (just make the sprite a single black pixel; I've done this and it seems to work fine) and untargetable, give it the beam weapon you want. Then make an invisible mission that the player has at the appropriate time (whenever the stellar is alive and visible) that places said ship in the proper system at the proper nav default to make it appear over said planet (so nav default 1 if it's the first stellar in the system, 2 if it's second, etc.). If the planet can be destroyed, set its OnDestroy/OnDominate bit to abort the mission that spawns the ship and OnRelease/OnRegenerate to start it again.
Limitations: You won't get the typical spöb behavior in that if a player manages to get landing clearance (either by bribe or mission), the planet will still shoot at them. Also, like I said, a player can only have 16 missions active at once, so even one permanent hidden mission can be a major restriction.
I'll keep mulling over other possible workarounds, but given that there's really no way to "trick" Nova into using beams (e.g. by submunitions), I'm not seeing anything else at the moment.