Aren't your vertical game snaps already vertical aspect resolutions? If you set preserve_aspect_ratio to true, you could run horizontal and vertical snaps in the same area and it would fit in there.
A visual editor is something i've worked on from time to time, but it will probably be awhile before I can make it happen.
Quick tip for code editing though:
function reload(str) { if ( str == "custom2" ) fe.signal("reload") }
fe.add_signal_handler(this, "reload")
This allows you to reload your layout with a hotkey, so you can make a change in code and reload it. This is handy to adjust your object positions, or do various testing. Only thing to keep in mind here is if your layout generates an error, it may not be able to reload without shutting down and restarting.