I think the question was asking for a path of arbitrary length;

Indeed. Maybe there is a better term for the property "directly or indirectly connected", isn't there?

Transitively connected?

in that case, breadth_first_search or depth_first_search would find the path.

The problem here is that we need to stop the search once we've found a connection. But neither BFS visitors nor DFS visitors seems to give us a chance to cancel the search. The discover_vertex visitor could have this ability (I slightly remember that I had the problem to cancel a search in BGL some years ago and I wasn't able to solve it by then too)

The most promising (i.e., least invasive) solution to prematurely stopping an algorithm has been to simply throw and catch an exception. Another trick was to give your visitor a reference to the queue and color map and then empty the queue and color any other vertices black to make the algorithm think that it's done.