Where is this idea that borg technology needs a "blood stream" to assimilate coming from? Yes nano probes are present in
the bloodstream, but it does not follow that it's required for the asssilmation process to occur. sophisticated nanoprobes could work with whatever material was presented - we've seen then adjust and change the structure of material such as the walls of the enterprise.
As I mentioned before, the founders were affected with a virus so must have cells that could be targeted.