[spoon]

The amounts can be modded, but the location must always be over a star...

The DevNull mod does this with Stellar Moons. Here are the areas you'd need to mod, using Stellar Moons as an example:

From components.txt:
-----
Name := Artificial Moon Placement Generator
Description := Provides the gravitational matrix needed to assemble a stellar moon.
Pic Num := 178
Tonnage Space Taken := 500
Tonnage Structure := 500
Cost Minerals := 25000
Cost Organics := 25000
Cost Radioactives := 25000
Vehicle Type := Ship\Base
Supply Amount Used := 500
Restrictions := None
General Group := Stellar Manipulation
Family := 3022
Roman Numeral := 0
Custom Group := 0
Number of Tech Req := 1
Tech Area Req 1 := Artificial Stellar Moon Construction
Tech Level Req 1 := 1
Number of Abilities := 2
Ability 1 Type := Create Constructed Planet
Ability 1 Descr := Will create an artificial moon around a star.
Ability 1 Val 1 := 3
Ability 1 Val 2 := 0
Ability 2 Type := Constructed Planet Requirements
Ability 2 Descr := Must have 5000kT of artificial moon components present at location.
Ability 2 Val 1 := 3
Ability 2 Val 2 := 5000
Weapon Type := None

Name := Artificial Moon Component
Description := Provides the ground material and infrastructure used in artificial moon construction.
Pic Num := 245
Tonnage Space Taken := 500
Tonnage Structure := 500
Cost Minerals := 5000
Cost Organics := 5000
Cost Radioactives := 5000
Vehicle Type := Ship\Base
Supply Amount Used := 0
Restrictions := None
General Group := Stellar Manipulation
Family := 3023
Roman Numeral := 0
Custom Group := 3
Number of Tech Req := 1
Tech Area Req 1 := Artificial Stellar Moon Construction
Tech Level Req 1 := 1
Number of Abilities := 0
Weapon Type := None

----
From PlanetSize.txt:

Name := Stellar Moon
Physical Type := Planet
Stellar Size := Tiny
Max Facilities := 25
Max Population := 10000
Max Cargo Spaces := 20000
Max Facilities Domed := 10
Max Population Domed := 4000
Max Cargo Spaces Domed := 8000
Constructed := True
Special Ability ID := 3

-----

That Last line (special ability ID) matches up with the value given to the "Create Constructed Planet" ability on the component.