Since kite-spar is far more rigid that PVC, one can often retain rigidity over a longer length with a much small diameter, ie. most 2-Hander PVC pipe needs to be at least 3/4" in order to pass the "whip test", but if you don't mind investing $9-12 a core (for a Diamondridge or other high-carbon material), you can use 1/2" or even 3/8" Kite-spar for a 2-Hander and it still won't whip.
The trick will be to find pipe insulation which fits the core and still meets the 5/8" wall regulations. This can get tricky if you're not using the standard 1/2" or 3/4" core. If you choose to go with a thinner core, you'll have to figure a way to either: find smaller circumference insulation (expensive); resize the foam you have (must have a sure exacto hand); stuff the gap in the core with liquid-foam insulation (will demand more maintenance and add weight).
One last note, when you're looking at which Kite-spar to buy, keep an eye on the Outer-Diameter (which will determine your insulation inner diameter), the total length (some sites only sell thinner spar in 3' lots or shorter), and solid vs hollow core (you'll prolly want hollow core in order to minimize weight).
PS. Brooksy, ¿I thought you'd foresworn UL as demon-spawn? Why the sudden shift?