I've found a product which I think will act as an alternative to hart's Flash Black. It's called Shoe GOO and should be available in the USA and Japan; a UK eBay trader is selling it too.
It comes in a rubberised black version and a clear one. more info at:
http://www.shoegoo.co.jp/lineup.html
I've just begun experimenting but it is supposed to be usable as an adhesive, for gap filling and I think just building up a tough rubbery layer.
I'll report back on performance anon.