Regarding Jeff’s XTB-XM, if the Harmony cannot send out X10 pre-set dim commands, a workaround would be to use X10 scenes. The Insteon 2477D Dimmer Switch, the Insteon 2334-222 and 2334-232 Keypad Dimmers, and several other Insteon dimmers, are capable of storing X10 scene addresses in addition to their primary X10 address.
For example, suppose a 2477D was set up with the X10 primary address A1 at 100%, and X10 scene address B1 at 50%. If the Harmony were to send out a X10 B1 ON command, that would be equivalent to sending out a X10 A1 50% pre-set dim command.