How can I T splice a 120V line in a lighting fixture?
As I interpret the code, no. That "T" splice isn't allowed, and not really elegant IMHO.

2011 NEC
110.14 Electrical Connections
(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.
...


Where the article says you could solder the joint, in your case it's shall solder the joint. As far as the taps being in a approved enclosure, I can't agree with the commentary. Just go to any lighting store and you will see lights that have fixture wires on top of a light that would be much larger than any typical box. However, I'm not a manufacturer, I just have experience hanging a ton of lights.