[Obligatory "if-it-works" clause]
Do trigger_changetargets change the end/start of an env_beam? If so, some clever use of those could decouple the beam.
I have conducted tests under the observatory eye of DiscoStu, and have discovered two things:
1) trigger_changetargets cannot alter the target of an env_beam. Disabling Smart Edit shows that neither the end nor start targets use the parameter "target"; this is likely why.
2) An env_beam set to target "player" neither follows the player nor targets him/her in any way. The beam defaults to targeting the map's origin.
Both these issues may have solutions in Spirit (indeed, the first can be worked around in vanilla), and while I could guess at how to solve the first, the second, far more significant issue I am less sure of.