Aah, cool 
I hope it helped.
I am quite familiar with the Bancor formula. I translated the smart contracts to C# to try to do some simulations.
I was fascinated by the proposed capability of Bancor protocol. It was almost too good to be true that it could be so powerful with such a simple algorithm. It turns out it was. Check this out, well written:
http://hackingdistributed.com/2017/06/19/bancor-is-flawed/
I don’t see it as a good enough solution. However, we can still be inspired by it for an idea such as yours, and there are plenty to discover in that area, the general direction of the concept is very cool.
Well, if we are going to reflect current valuation of a token, then we need some continuously updated price.
If Bob sets price in SAFECoin, then there must be some exchange rate available for the other coins that might be used for payment. This rate better reflect current market valuation as close as possible. And so a price must be set somewhere, and it must reflect the current estimated value by the market.
What this something is, could be a matter of research and invention. The traditional way is the exchange, where supply&demand from buy and sell orders sets the price, and in case of various exchanges, evened out by arbitrageurs.
Anyhow, each vendor could fetch the price from somewhere. Additionally a preference for certain tokens - set by Bob - could also be used. (preference set by wallet providers would not be good, as you mention).
I need to do more research to come further in my thinking here
Could be a good start for a topic.