Doesn't complicate anything. I called in to register a stripped sa15.7 build with a 20 inch barrel. They just added a child frt entry to the existing sa15.7 entry to cover the 20 inch barrel. Wasn't an issue. Did it over the phone. Mailed out the new reg cert.
The sa20 parent FRT entry currently has child entries for 508mm (20in) 5.56 barrels.
The sa15.7 parent frt entry currently has child entries for 387mm (15.7 inch) 5.56 factory, 292mm (11.5inch) 5.56 customized, and 508mm (20in) .223 customized barrels.
If you don't have one of those barrels, just call them up and tell them the new barrel length in mm. They'll add another child entry to the parent file.