I had a weapon concept almost identical to this almost a year ago, buuuut I never got feedback for it so, meh.
Anyways, the concept is sound. My theory on it is to have it's arming time reduce by x amount of seconds every second after holding right click.
So, effectively, let's say it takes it 6 seconds for the projectile to reach it's arming time. Cooking the weapon every second reduces the arming time by each second. So, holding right click for 4 seconds would mean that it would detonate in 2 seconds in it's travel time.
That way, it works effectively like a grenade, cooked along it's timer in order to give greater control on it's detonation.