Not sure it makes sense thermodynamically to deal with waste heat that way (if you are transmitting "waste heat" in a narrow beam, you are basically just transmitting energy in a narrow beam, and so it's not waste heat anymore -- you can get useful work out of it).

edit: I suppose the question is: what % of the star's outgoing energy can we harness in principle, such that waste heat is hard to tell apart from background, and we completely hide that the star is there. For example, in the limit, if you just used a little of the star's energy to redirect all the rest into a black hole, will the waste heat just from the redirection effort be detectable? If so, we can't hide a star, the best we can do is not use too much energy, so the star looks like a normal star with no life in the system (but evil aliens can still come and check it out, since they know a star is there). If not, maybe we can harness some bigger % on the way into the black hole. If so, what % is physically possible? I don't know.

I feel like physicists already worked out that you can't hide stars, but I don't know the literature.