I've played open D and G (the Keef one with the D on the bottom) and have been messing with C6 on my lap steel recently, though I have everything turned down to A so I can get some playing room in and around the D position on the 5th fret.
Thing is though, when I'm in a corner and have to come up with something fast, I'll usually just do the
slide parts in standard tuning.
I read that Howe tunes his steels to standard, and I can...sorta understand that.