I would like to add another show of interest. I have a fair amount of coding experience, but not in Lua. Checking both your mod and the Factorissimo2 codes, it seems like a Factorissimo2 factory contains information about the surface and position of where it is sitting, while yours sets positional and surface data when the teleporter is placed. Maybe a recursive check of the factories' surfaces until the position on the surface of nauvis is returned. If i am reading the code right, this check will only be performed when the entity is first placed, which won't be too much overhead, hopefully. This does mean intrafactory teleportation would essentially start and end at the same point on nauvis, which might cause some problems is the cost calculation. I haven't gotten to that point in the code yet. Finally, if the factory is relocated on Nauvis, then those calculations would have to be redone... Now that I am typing this out, I am seeing some of the major difficulties, and I only have a brief layman's understanding of your code.
In any case, thank you for the mod! I am a huge fan of it, and I just want to carry around a 5kspm base in my pocket, and your teleporters are the only throughput fast enough for that goal ;)
