Well, it turns out GIMP has that feature too, and has had it for a long time. The reason it had never worked for me is that it only works if you click on one (any) of the tool buttons. I was clicking in empty areas of the toolbox window, because it feels weird to click over a button when I don't mean to use that tool.
Now that I know to middleclick on a tool button, middlemouse open works great for Unix paths, file: URLs and even remote URLs (assuming you have Open URL working, of course, which on some systems may require installing gimp-libcurl or gimp-gnomevfs).
Nice! That'll save me some gimp-remote calls.
[ 16:21 Feb 26, 2008 More gimp | permalink to this entry | ]