Effective the night of Thursday, January 20 (morning of Fri. Jan. 21), the DH backups you set in the Lineup Depth Chart will (finally) work, the same way as all the other positions do. This is for all games under the central SOM Online site (2007-2010, ATG, 1986, 1999, Btt90s) only.
You may recall I tried and failed to deploy this added functionality back in September. This time it's been tested more robustly and already deployed for a number of live leagues successfully.
As a reminder, the reason you could never set DH backups before is because the system uses the same underlying code as the CD-ROM game, which didn't support DH backups either. And it is/was not a straightforward upgrade (to say the least). DH backups will now work as the rest of the positions do in the Lineup Depth Chart, but I haven't made it smart enough yet to choose the worst-fielding guy in the field to switch to the DH after the backup has been chosen. Hopefully I'll get to that eventually. But for what it's worth, it works as intended.