I tried the flag approach this year, but I have to say my success was ZERO! Perhaps it was the time of year, perhaps it was a consequence of the location of my campsites, perhaps I needed a bath ... in any case, I didn't attract any visitors.

On the flip side, I did paddle up to one camp site at about the lunch hour.  I was interested in getting recent, first hand information on a nearby remote portage I knew the group had taken (from observation).  I got the distinct feeling this particular group had NO interesting in visiting.  I had to hail them 3 times before I even got an acknowledgement and then the answers were as brief as possible.  I quickly bid this group a good day and paddled on.  

On the other hand, I found most groups I encountered on portages willing to stop and chat for a few minutes.  Most encounters on the water were limited to a quick hello while passing.  

So, if there is a secret to having folks stop in for a "cup of joe," I obviously haven't found it (although I'll keep trying).

dd