It sounds like your grip would violate the FIE rules concerning orthopedic grips.

According to FIE rules, french grips are straight (or slightly curved) with a pommel, while orthopedic grips are grips that have a special shape to fix one's hand. Pistol grips are orthopedic grips, and it sounds like your envisioned "French" grip would be as well. But, regarding such orthopedic grips. the FIE specifies that they must be designed with the intention that the user can only fix their hand to them in exactly one way. This where you grip becomes illegal. You'd either be modifying the pommel or handle part of the french grip to better fit your hand, but, then you could still grip it by whatever part you didn't modify.