playing slide in a different key

Perhaps I misunderstand your question, but yes,- you can play any scale or mode on slide/steel guitar, regardless of tuning.

Steinar

with open G your D chord is at the seventh fret.

Yes, to play in a key other than the open chord key, just look at the frets the chords for the desired key are found on and work around those. You can end up with some interesting "voicings" where, for example, you may play the I chord on a higher fret than the IV and V. The open chord may or may not be useful in your "new" key. That's not going to put you into a different mode, though, because the chords at each fret are still major chords (if you're in a major open tuning.)

yes. and to add interest just play the triad.
those three notes that make up the basic chord.
look for the notes on different strings.
play those three for another voice.

also, slants come in handy. I discovered a I V VII using slants.

yet another voicing.