Cool explanation, I like it!
You can see it as borrowing from yourself. To always keep bitUSD or other at parity, BitShares network must keep the collateral just as you describe. The house is your bitshares (BTS), and you are the bank! We've been talking about it a lot past few days, if it would be better to place a restriction so that once a position is open, it must be closed before adding to the debt. More collateral can be added and that's it until it is repaid - not the current system of 'borrow bitUSD, buy BTS, borrow bitUSD'. This could prevent people from continually go into more debt when price is tanking in a market downward movement such as we saw just the past week. Many experienced traders won't like it, but much more money would stay in the DEX if you think about it. Thanks to @murda-ra of course to a lot of this thinking.