Is soldering copper to aluminum wire allowed under NEC?
You mean like a rattail or Western Union splice, Al-Cu?
Your first problem is that soldering in general is so alien that an AHJ is likely to reject it out of hand, even if it were all Cu.  The ruling would likely be &quot;You can solder if you can show how every other splicing method is impracticable&quot;.
But when it comes to Cu-Al splicing...

NEC 110.14 Electrical Connections. Because of different characteristics of dissimilar metals, devices such as pressure terminal or pressure splicing connectors and soldering lugs shall be identified for the material of the conductor and shall be properly installed and used. Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar conductors (such as copper and aluminum, copper and copper-clad aluminum, or aluminum and copper-clad aluminum), unless the device is identified for the purpose and conditions of use.
(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.

The sections I put in bold are in contradiction with each other.   110.14(B) requires direct contact between the Cu and Al wires, yet 110.14 forbids it.
The path to soldering is plainly identified in 110.14 with &quot;soldering lugs&quot;.