I fully endorse the mermaidapault in any and all applications.
The main problems with a mermaidapult are:
1) It can't fling creatures up z-levels, only items. So you can't fling them out of the river easily.
2) We can't control the direction, it flings randomly.
I'm not saying it's impossible to use a bridge in the process, just not the way you would use it in real life.
As for merefarm-in-a-river, pressurized water doesn't push creatures/items, only diffusing water does.
I am trying to catch some mermaids now, but I just embarked and can only see 2 whales. Oh, well, as a first try that'll do. Also finding a site with magma (for easier in-sea building) and a river (for a farm like you proposed) on a shore(which can't be entered into the site finder) would've taken some time. Just magma on a shore is easy.
--Edit: If we drop merepeeps down a lot z-levels so they explode we get more bones, right?