So I see the challenge now. Normally switching something on and off, you switch between two fixed values, like 0 and 1. In this case, you are switching between 7F for off, and another variable, the channel number, for on. I do not believe that duplicating L or V totally solves the case - it fixes the ON case, but not the off case.
This looks like an ideal opportunity for StreamByter.
And L, while termed channel, is just a single byte variable that can have any value up to 7F. When you change the L parent control to type "channel changer" - do not accept the option to change the control to 1-16, you want it set for values up to 24.
I would make a sysex control, 43 10 4C 08 L 04 V, use the values 7F for off and 7E for on. (The 7E is what we are going to trap in SteamByter and change to the channel number.
StreamByter output code should look like:
# Template F0 43 10 4C 08 L 04 V
IF M0 == F0 43 10 4C # Header match
IF M4 == 08 # M4 match
IF M6 == 04 # M6 match, so work to do
IF M7 == 7E # On message
Ass M7 = M5 # Set value to equal channel number