UPDATE:

Called CARB at 800-242-4450 and was transferred to Angus at
916-445-4686 who discussed with me the legality of gas caps:
http://www.arb.ca.gov/html/contact.htm

Angus says that the cans aren't tested in the sun, but the
"system" is certified for permeability after 140 days in an
enclosure or after an accelerated 30-day test at 100 degrees F.

It seemed to him that gas cap 'should' easily meet those
permeability requirements, as long as it's made of similar
material.

Angus said the cap must hold the ~13 psi above atmospheric
Reid Vapor Pressure of California winter formulation gasoline
(which has a higher vapor pressure than summer formulations);
but that RVP & permeability requirement seems easy enough for
a gas cap of the same material as the current spout.

In addition, the spout must be spill proof and child proof;
but again, those requirements would be met by use of a cap.

The only requirement of the spout that the cap can't meet
is the auto-shutoff requirement; but if the cap is used just
for storage, then that requirement wouldn't be applicable.

Interestingly, Angus said there is a provision in the specs
for a "secondary opening" (e.g., a vent!); but he said none
of the cans certified for California have opted for that
secondary opening.

Angus said these 5 manufacturers are certified in California:
1. The Plastics Group
2. Scepter
3. Midwest
4. Great Outdoor Products
5. NoSpill

Of those manufacturers, the first three have the lions share of
sales in California, according to Angus, and the first is the
one he thinks who bought the Blitz operation in July of 2012.

That explains why I can't get a hold of anyone at Blitz
to affirm the threads on the gas can caps. I'll work on
The Plastics Group next, to see if they sell just the gas caps.

Q: Anyone have a 411 on "The Plastics Group"?