The Bluegrass Waltz RealTracks is a faster style, and Even feel, so wouldn't be useful for a slow swing waltz.
Since we don't have a slow Bluegrass Waltz in RealTracks, the closest thing (other than MIDI style BGWALTZ) would be to

- use the MIDI style BGWALTZ, but replace the following 3 players: (using the RealTracks Assign dialog)

Bass track, replace MIDI with this------> 777:Bass, Acoustic, PopWaltz Ev 085 (A:simple)
Piano track, replace MIDI with this------> 824:Guitar, Acoustic, Strumming Country Waltz Sw 085
Guitar track, replace MIDI with this------> 822:Guitar, Acoustic, Fingerpicking Country Waltz Sw 085

And mute the drums.

That's a nice sounding slow swing bluegrass style, (though a MIDI mandolin and no banjo)
It sounds like this http://nn.pgmusic.com/pgfiles/jazzu/misc/=BG_Waltz.m4a
I've uploaded the style and demo song here http://nn.pgmusic.com/pgfiles/jazzu/misc/=bg_waltz.zip
..

Have Fun!
Peter Gannon
PG Music Inc.