Steve,
I have yet to see anything in NEC authorizing twisting + solder beyond 90.2.(C). Not that 90.2.(C) isn't plenty for most jurisdictions, but it's not the same as actual code allowance.
110.14(B)specifically permits a soldered connection.
(B) Splices. Conductors shall be spliced or joined with splicing devices identified for the use or by brazing, welding, or soldering with a fusible metal or alloy. Soldered splices shall first be spliced or joined so as to be mechanically and electrically secure without solder and then be soldered. All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors or with an insulating device identified for the purpose.
