I've had XM since six months of them launching service, there was no dedicated 20s music station. This is the channel line up from when I first got service in the Spring of 2002. There's Top 20 on 20, which was a Hot AC station where listeners would cast their votes for the top 20 songs, and they were played ad nauseam.
On Sunday evenings on Real Jazz 67 they have Traditional Dixieland Jazz beginning at 9 PM ET. That's probably the closest to 20's music you'll find.