Super Saiyan said:
Given that boss Old Kaioshin unlocked all of Gohan's potential + more, would it still be possible for him to get a rage boost?
The Old Kaioshin unlocked his Potential at that point in time, not forever.
Potential is always either growing when used or decreasing when neglected.
Yes.
SuperSaiyan FullPowered broke passed its Potential with a Rage Boost.
Broly beyond his Potential & continued stacking Rage Boosts on top of Rage Boosts, & went beyond with by getting Enraged & going SuperSaiyan.
Vegeta went beyond his SuperSaiyan Blue Limit with LimitBreakerBlue.
Goku did with Ultra-Instinct.
Enraged states from what the Material portrays can unleash even more Potential Power one has/is capable of at a current time.
Hence Vegeta vs Beerus.
IF Gohan has hit a limit in his Ultimate Form there's no reason I see getting Enraged would not be privilege to him as SUPER has apparently removed that cap.