To operate this concept, you cannot rely on a third party wallet, you have to be a full fledged wallet provider yourself. If you rely on a third party and one fine morning the site is gone with all your (rather other's I would say) coins, then you know where you have to head to.
For the system to work, it would be necessary obviously that the participants use different wallets and exchanges. A good bet would be a "rebalancer bot" that does the rebalancing in an automatized way, is platform-independent and extensible by the API commands of the different exchanges and "fixed-rate shops" like Xapo.
It would be also interesting to integrate Bitsquare and Coinffeine (if it still exists) and perhaps also "fiat-backed" cryptocurrencies like the BitUSD. This way, single points of failure could be minimized.
PS: Just found
this "portfolio rebalancing" bot that already uses the strategy since the year 2013. It was conceived for defunct exchange MtGox but surely can be adapted to be platform independent. Maybe there is a newer bot available.
If we are to "impose" guarantees for x prices at y moments almost every price observation algorithm should have to include big merchants and at the same time BitPay, Coinify and other api providers should agree at the spreads of the price (eg at 19/6 UTC 00:00 the price stands at 2000$ & we guarantee this value until 12:00) but again the interval is big and they could lose or gain considerable amounts of fees if they are to convert their coins to fiat asap.
In
my idea, the system would be totally decentralized, so every merchant on every platform could choose the price that he considers convenient to give his price guarantees. BitPay would only have to enable the option for merchants, but the provider itself would not need to guarantee nothing (because then, the backing mechanism would be fiat-based, not backed by "goods and service"). A good idea would be to use a price index like the one Coindesk uses and computes various exchanges and maybe moving averages, to avoid short term spikes.