I DO like the idea!~
However, from the other side,...the only concern that could hold anything back would be is that people that fish 'inland' (in general) tend to keep their catch/ location, etc a secret, more so than the Saltys.
Salty Dogs seem to be more than willing to share their Lat/Long numbers, location, numbers of fish, etc. I've seen a few different posts and opinions about folks inland on the rivers, etc, not letting other know they have a fish, for fear someone might take their 'spot' etc.
and please, these are only observations and generalizations. I realize not all are like that, just saying that seems to be the overall observation.
just something to think about when deciding to make the flags. Personally, I think its a pretty cool idea. I know if I'm on a boat, cruising by another boat that's trolling, and has their Salmon flag up, ...so goes my thumb!, and probably a "Woohoo!" or a "Great Job".