Interesting tool, forgot this existed!
Got interested in it and I did a little exploring of this and I don't think this tool will be useful in this case as the tool only extends lines that are perpendicular to the line its extending to. If the gaps in his river are between u/s and d/s polylines (Martin never says where they are) then this tool will not generate the expected results as the ends with be parallel.
If he uses this to join a tributary to a main stem then it should work but it does not introduce a node so he will need to be doing that afterwards.
Still I can see this being a useful tool for other types of processing.