Anyway, given the interest Mr. P wrote out the specification a little, for this thing provisionally called
BTC-UXP. All comments more than welcome.
I did a quick look&see. It doesn't seem to have the protection against placing duplicate orders in case of transport failure/timeout. At the minimum all the imperative verb calls should have an OrderID argument that needs to be unique.
There may be some sort of replay attack made out the above flaw, but I don't have a motivation to delve deeper.