IMO that is conflicting concerns, making timestamps responsible for fee changes is adding complexity to something that should be simple aswell as makes fees depend on the granularity of those timestamps...
If you can store metadata somehow in your transactions all you need to do is store some json/xml/whatever data format and read that data to set your fees, what I did was create a data alias that I own and I can update, and I update with a bot whenever the average exchange rate moves by greater than x%. This creates a "fee pegging" based on any currency and then you can market your fees to say something like $1USD per creation of an offer for example so its easy to understand for avg joes.
You bring up excellent points. I was thinking about using Bittrex's JSON API to convert Sling to BTC then USD or Euro but with the current volume being so low, the price is almost meaningless. For example, if you try to buy $400 worth of Sling right now to make a purchase, it would cost much more unless you waited for buy orders to fill. With that said, your method is still better than the changing fees but that is where I got stuck with the JSON API.
You'd need to store it in some meta data(in the blockchain) that is provably linked to a key you own such that only you can update that metadata, using the namecoin pattern of aliases you can do something like that by spending a tx of a coin you own with another tx (coloured with the same op_code type) and saving it in the alias DB in this example. This way you can update based on a bot which pulls from bittrex or wherever.
I've read the devs code for sling and it uses a different approach, I see you are doing multisig tx's for escrow and whatenot but I also see p2p messages to broadcast offers, so im not sure those offers are coloured tx's... however you can send multiple broadcasts of those offers if you can prove that you are the one who can sign that offer in a new broadcast or tx. (offerupdate)...
in general offer updating is essential anyways to change things like description, qty or price.
