One thing that could cause trouble with this is that at any time, if you hold BitUSD, you're subject to having your BitUSD automatically replaced with XTS, because margin calls can happen at any time. If I'm shorting BitGLD using BitUSD as collateral, my BitUSD could be replaced with XTS with no warning. There's nothing inherently wrong with that - collateral is collateral. But it adds some complexity that might not be good.
I'm not saying it can't be done, just trying to talk through the issues.