Another way to play lead style slide in different keys is the good old capo.
This key of G piece is based on the Chicago blues sound typified by the Muddy Waters Band.
Little Feat featuring Lowell George on slide also used this capo technique a lot.
Basically the capo moves everything up 3 frets and you play as if in open E.
Notice that in this piece I am not damping behind the slide and also that I go right up the fretboard and even over the edge ~ my guitar has only 21 frets so where you see fret 22 marked on the tab I'm kind of guessing where that is.
If you have a good mental picture of where the notes are and you're brave enough you could go even higher - i.e. you're not actually limited by the frets.
This is Standard 12 Bar variation #1
In the first 4 bars instead of doing the standard
| G / / / | G / / / | G / / / | G / / / |
| G / / / | C7 / / / | G / / / | G7 / / / |
The trick is to apply pressure on the 4th bar ( the G7 bar ) — notice how I do this on both verses by using triplets (1 2 3 - 1 2 3 - 1 2 3 - 1 2 3 )
You can then ease off for bar 5 to get "light and shade" into your improvisation.
Don't be afraid to use some chords in your lead improv. - with a bit of vibrato chords can sound very effective.
The root G is either at the capo ( fret 3 ) or the 15th fret.
The 2nd chord C is at the 8th or 20th fret
The 3rd chord D is at the 10th or the 22nd fret.
If you know where G, C and D are in both octaves ( in conventional tuning ) this should be fairly straightforward.