I used an extra solenoid for the 24 up. It is shut off by a limit switch. 12v up and down left blinker switch, 24 volt up and 12v down on the right blinker switch.
Now the 24up is triggered by the ECU also, once the TP sensor reaches 90% and the RPM reaches 4,500 rpm. The ECU grounds the relay.
An extra trim switch for just 24 would be the ticket, unless you're using a timer. Ultimately, 12 down and up and then just 24 up wired separately for a lake boat would be the ticket.
I use a third solenoid also. Mine is just wired thru the timer. All the buttons on the wheel are still 12v. You just have to rig it as a stand alone system from the 12v stuff
I've had good luck with the continuous duty 24v solenoids like they sell for forklifts. Pretty heavy duty pieces. By the way Tom, why would a mild mannered never pushes the throttle all the way down skirt chasing SS owner ever need 24 up for anyway. Any potential victims on the river up there giving you trouble?