The first try ended up looking like this:
I made a makeshift decaleur by inserting a threaded rod into a threaded hole on the Surly rack, used a piece of metal and couple of nuts to secure it to the rack little higher. The bag loops are around a piece of old straight handlebars, also secured with bolts. A rubber band connecting the bag and the threaded rod is holding the bag put, and on the third picture there’s the red rubber band giving some extra stability on the front.
The system worked pretty well on a two day tour in Estonia. The bag could use a loop on the bottom so there wouldn’t be need for the red rubber band. The threaded rod should be substituted to flat piece of aluminium to get more sideways stiffness.