If there is no natural space in the puzzle for the two arrows, you can just reformat the grid with an extra gap. I definitely like this arrow line idea.
Then this will be a very convenient and easy answer key. Definitely easier than counting double bridges.
Just want to note one downside of this suggestion, compared to the version used on PR (which was to ask for a sequence of N numbers 0/1/2 counting the "horizontal" bridges in a row of N+1 clues). Namely, that the suggestion here has no fixed length. Some form of checksum like a fixed length or sum is very useful to avoid typos.