This post was submitted awhile ago, so I don't know if you figured out your idea on his masteries yet, but I think you are greatly under estimating the value of Fervor of Battle. At lvl 1, I think it's like 1.3 bonus damage per stack, and increasing as you level. Now, you don't have to actually CONSTANTLY auto attack to keep these stacks, it has easily a 5+ second timer on it before it goes away, if not closer to 10 ( can't remember exact time ). Now, if you play Renekton a lot, you will notice that his W applies on hit affects for each strike; 2 normally, 3 when empowered, and Fervor of battle is an on hit effect.
Not only is Fervor of Battle great for extended fights; e.g. bruisers, but it combos amazing with Renektons W. Let's say you are level 3 and ready to do some damage trades, at level 3 Fervor adds 2.16 damage per stack, or 21.6 damage at max 10. So the damage bonus alone on Renektons W is 42 or 63 damage depending on if empowered, not to mention any other additional auto attacks that you do, which is a substantial amount of damage at that level.
I personally would use this mastery on Renekton; even though he is a physical caster, he still uses auto attacks, especially in prolonged fights ( where Fervor excels the most ). At lvl 18 Fervor adds 84.6 damage per auto attack when max 10 stacks, ( spells and auto attacks give a stack, even on minions/monsters ), so you are doing an additional 85 damage per auto attack level 18, and if you went bruiser, that's roughly a 30% damage increase on your auto attacks, and an additional 240 damage on empowered W.