The 400lb guerilla in the room is the fact that to even consider
mounting such a mission requires two active launch pads. One for Falcon
9 and one for the Falcon Heavy. It has been posited that too much
oxidizer would be lost from the upper FH stage if it was launched first
awaiting the turnover of LC-1 to a crewed F9. If the missions were
reversed it would still require a quick turnaround in order to get to
the orbiting Crew Dragon with enough endurance to complete the mission.
We don't really know what that number is. (Well I don't anyway).

There has been talk of upgrading SLC-40 to handle Falcon Heavy I
believe. Maybe after that has been done?

Dave