Interesting thread. It's got me thinking about tubeless bicycle tires. The sealant inside tubeless bike tires maintains its liquid consistency. It's definitely heavier than the tubed alternative, but has advantages. For example, if the tubeless tire is punctured, the sealant plugs the leak before the tire deflates.
A liquid sealant would cause a kite's balance to vary as it flies across the wind window. You might be able to "even things out" by constantly looping the kite (like a rotating bicycle tire).
You'd also have to come up with a new deflate system. Otherwise, you'd end up with a face full of sealant at the end of every session.
Surely, somebody can solve these problems???