Time Capsule as scratch disk??

Wirelessly? Regardless of app, you could try, but it would so damned slow as to be almost pointless.

Best connections for external scratch disks are eSATA and FW800, and even then they're not ideal. Photoshop scratch files can get up to 2-3Gb in size, sometimes more in extreme cases... video would be just as, even not more, demanding. Time Capsule is a kind of restricted backup solution, that's all.

Better to upgrade your internal hard drive.

 

Even over wired, the no-brainer (literally) nature of the TC in that it's not got the brainpower to shift Gigabit traffic at anywhere near Gigabit speeds will make it very slow. Best to follow BV's advice.