Hi Paul,
We don't currently know of any customers that are actively using K2 and DBXL. We have scheduled some time in September to work with a K2 consultant to vet the scenario and write a blog or two on it. There is nothing that suggests it won't work.
In general, K2 has some good stuff with SmartObjects but it suffers from various issues. K2 does provide some good quick and dirty ways to get data in and out of InfoPath but it only works well when the data is flat - it cannot work with collections wells. That's what DBXL is good at - arbitrarily deep collection wells. Same reason why DBXL works well with SharePoint (libraries and lists are flat and can't handle collections).
Good luck with your project.