I think that exploiting the fact that trade fees are taken in one asset and returned in another is a terrible way to handle this. In fact, that particularly exploitable "feature" should either be removed, or made an optional permission/flag for asset creation. Instead fees should be returned in the same form as they were taken by default. And if an issuer wants to withdraw from the fee pool, that should be made an optional permission/flag on the asset as well.

I'm not sure how they implemented it, but uphold.com also provides gold- (and silver, platinum, and palladium)- backed crypto assets which they claim are 100% backed by physical assets.