I can't decide if my implementation is actually wrong, or if it just looks rubbish. What do you think:
Subtle star filter. You can just about see the blades, but they aren't very long.
Medium star filter. The blades are more pronounced, but the middle section is far too bright.
Crazy star filter! You can see the true length of the blades here, but the colour balance is just so wrong it's unusable.
For reference purposes, this is roughly what I'm hoping to achieve:
Kawase's Light Streak filter uses a slightly unintuitive set of sampling points across multiple passes (17 for the above images). It seems to make sense, but as shown by the last image, it samples a lot of pixels that don't actually contribute to the final image. The blades in the first image should by the same length as the 3rd image - but they're being attenuated out so as to disappear.
I think I'll just have to keep working on it - but given I now understand Kawase's approach I might just go and implement my own system...