But, it left me wondering how hard it would be to implement something like that as a plugin in OctoPrint or if it's already been done. Does anyone know of an existing plug-in like that?
I know about the Custom Control plugin and that would give an approximation of most of the capability, but not the graphical layout of the print bed for direct selection of an XY position especially.
Plugins can do whatever they want really, there's a lot of customization available. They can even replace the whole UI if one wanted to completely clone the look of PrusaLink... Don't recommend as Prusa will probably come after you.
However, it would be possible to do the graphical bed with the XY position in a plugin. The bed level visualizer plugin for example allows you to move directly to a point on the mesh, this kind of thing is definitely possible - it's all frontend work just sending the relevant commands.
Nothing exists already, but I don't think that would stop it being implemented - as long as it isn't cut & paste out of Prusa and the code is implemented from scratch it is a sound idea.