Yes, in fact some of the premade maps like Ancient Gifts have artifacts at all the homeworlds. I thought they used "Always" and "NoOwner" as well, but it maybe they are using "PlanetOwner" or whatever it is instead of NoOwner. It's been a while since I've looked at it. If all else fails you could open that map in Galaxy Forge to check what they are doing.
Actually, I tried doing that before I posted. What ancient gifts does is that is has a custom template built specifically for wormhole travel, which I have not seen on any other map. I suppose I need to dig around and try to figure out how to make a custom template. I suppose maybe that is what the edit button under templates is for?
Also just a miscellaneous note, there is actually a small problem when spawning artifacts on non-homeworlds. Lets say you place the kinetic intensifier artifact at an asteroid next to your homeworld. If you leave the artifact density at something besides 0, its possible the map might choose that asteroid to randomly place an artifact. In this case it is possible that the kinetic intensifier you placed will get replaced with some other random artifact, as you can only have 1 artifact per planet. So you'd still find an artifact there, just perhaps not the one you intended.
And that fixes one of my frustrations, thank you, that helps me out immensely.
I've got a couple other general questions, if you feel inclined to answer.
I noted that you said that a planet can have 3 bonuses. Firstly on this I've noted that Greenhouse planets seem to keep the 3 I put on them and sometimes randomly end up with more. I'm sure this is because as you mentioned with the density levels it could change things. The question I have is; counting the TEC Homeworld as one bonus, is there a reason I can only get one more to spawn on a capital world?
Related to your E4X galaxy forge; I'm having difficulty spawning the ancient entity starbase constructor. I'm using the "always" and "no Owner" Is it possibly being eaten by the artifact or planet bonus densities?
And also likely related to E4X; It might just be a difference between Terran home, and Terran. But my capital world with full research/social spec caps out just under 900 population or so. but all other Terran worlds I've taken with the same upgrades get over 1,400 without any planet bonus to make the difference. It's not really a problem, just something odd I noticed.
Speaking of E4x, at least with the one i have (might not be the latest) Fortress world planet bonus is crazy OP. 50 extra mines with what you did to them? I haven't looked close enough to see if the mine DOT stacks, but still, the coverage is outstanding.