as far as i understand it, ecash has nothing to do with bitcoin at all. you can say you back it with whatever you want as it is backed by nothing at all, you can create an endless amount of ecash.
Discussion
There are some Proof of Reserve schemes that people have been kicking around for a while. That would be hard proof that the mint has the BTC (or whatever).
Also Calle has this Proof of Liabilities scheme.
https://gist.github.com/callebtc/ed5228d1d8cbaade0104db5d1cf63939
Ideas... none of them are implemented anywhere yet as far as I know.