high voltage e-load

[SeanB]

It will destroy it long term, but spare magnetrons are just a run to the nearest scrap yard away and pay $1 for them or even free if you look around on trash day.

[richard.cs]

AFAIK, oven type magnetrons use thoriated tungstent filaments.

In that case they should work indefinately* That's the method normally used to control beam current in x-ray tubes and depending on how constant a current is needed may actually be suitable for the OP's requirements.

Still I get the impression his boss may not like it.

*well, for their normal service life.