As far as I understand it, a holorifle's projectile is a bolt very similar in composition to the Sierra Madre holograms-- essentially, it's a "solid" photon blast, which explains why the shots behave more like plasma bolts than laser beams.
I still don't know how I feel about its propriety within the Fallout setting, but I think any OMGTEHLOOTZ hokiness is at least slightly mitigated by the fact that it's one-of-a-kind junktech, cobbled together by a lone mad genius in his travels and based on extremely rare old-world knowhow that he had quested far and wide to obtain. Also, your player character had to go through hell for the right to tote it around the Mohave. Looking at it in those terms, I tend to think it sort of fits, and I can almost even envision its inclusion in FO1 or 2.