I love how simple your script is, but I think that implementing as a plugin it can be more flexible (configuration, toolbar integration, etc).
Yes, toolbar integration is an advantage. Adding behind-the-scenes behavior works fine in Lua, but adding things to the interface really needs C.
Is it feasible to copy the integrated diff viewer from the geanyvc plugin? That would be really useful. I don't know about you, but the geanyvc diff behavior was what gave me the idea for compare.lua.
I've uploaded a new version, by the way, replacing the hacked-together file-choosing alert with a proper choice dialog.