Back in the day, my corps used both mellos and French horn G bugles. Assuming manufacturers still make marching French horns, is anyone using them?
As an aside, this may not be practical on the field for different reasons, but imo one of the most effective uses of French horns in orchestral music is in the low register. Even band literature rarely exploits this tonal color. It's not that low brass already play those notes, it's a different color in the palatte.
Low register French horn is just a different sound than bari/euph/trombone/tuba.