Ok let's do some simple math for the shaft you're looking for...
3 oz = 85,02 gramms. I've got a cf tube 30" long recently for some experimenting and its weight is 50 gramms with wall thickness 0,9 mm at the tip end and 1,25-1,3 mm at the joint end. I don't think Revo c.f. blank is significantly lighter. So 85 - 50 = 35 gramms that is how much is left for the phenolic insert at the joint end (if you want unilock insert in there add 10-11 gramms more). I guess you'd like it's filled with some foam but not the air so to feel better while shooting but in reality forget about it if your preference is 3 oz shaft...
.5oz (down from the advertised 3.5) is a lot in such a small area for a shaft, but yes, it would depend on the filler _if_ used. Should be doable without the filler, but I can imagine the design structure becomes much more complex.
Of course, what does shooting with a hollow c.f. actually feel like? I'm not too sure it would feel much different.
But yes, probably not coming anytime soon. I like light but long cues. Looking at Go-Custom, a 69" cue at 15oz should be somewhere around the limit Go-Custom is advertising with all c.f. (if they made a 40" butt, unlikely). Of course I'm in no way accurate enough to shoot with a 15oz, so the problem of adding a weight bolt chamber could come into play (but you could print a heavy ABS adapter, but that would limit it's position within the cue).
I don't know though, honestly, would shooting with a hollow c.f. shaft be much different? I'm starting to lean towards the opinion it wouldn't... much.