Currently currencies are limited to three-character codenames, so although this can be done and will work it will probably be useful to establish some convention(s) that will help people to not get too confused.

For example your examples seem to amount, in general, to "store credits", like airmiles or giftcards in a way. Thus it might be better to use a code for them that is different from the national cash they might happen to be useable on par with "at the store". So maybe instead of having lots of "different" IOUs labelled as USD most of which are not really someone owing an actual USD but, rather, someone owing in store credit, in other words goods or services, whose retail price happens to be in USD, it might be good to use USC or USX or whatever, the three letter code conventions only use two for the nation, deliberately leaving one for distinguishing between variants (such as day old dollars or tomorrow dollars or some such strangeness I think).

In general though, ripple docs already say that it is okay for a gateway to refuse to cash out people who are not actual members of that gateway, so that gateways can insist on knowing the customer and having anti money laundering data about that customer and so on. So we are told up front already that gateways are not under any obligation to actually make good on their IOUs to just any Tom Dick and Harry. Since that is already said, why not take it a step farther and say "oh yes I am a gateway, but only to people who buy my merchandise, and only up to the actual amount, per such person, that person spends at that moment on my merchandise".

I just think that would be a heading toward being kind of sleazy as compared to right up front setting your IOUs as not being USD but store credits, so people don't end up confusing them with actual fiat debt.

-MarkM-