The ring of sustain ability also confers level drain resistance.

Assuming an implementation that lets monsters wear rings and lets you pick and choose what your pet equips: You can have your pets wear an amulet of unchanging to make them immune from being polymorphed or shapeshifting; possibly the rings of sustain ability could protect from both as well, and having them wear protection from shape changers could be used to prevent them from shapeshifting, if they are a shapeshifter.

Wearing a blessed ring of sustain ability lets you exercise attributes, but not abuse them. A cursed one lets you abuse attributes, but not exercise them. It continues to block all other sources of ability score change. If wearing two rings of sustain ability, determine what happens by averaging the beatitude of the two.

Sustain ability also gives stun and confusion resistance, and prevents you from studying books.

Merge the rings of sustain ability and slow digestion into a “ring of stasis”. Double-edged effects: slows down digestion like normal, but also slows down healing, energy regeneration, timer effects like confusion and sickness, any timing-out intrinsics, prevents exercise from happening, blocks or reduces to 1 effects that directly change attributes.

If attribute changes from exercise and abuse are blocked due to the ring of sustain ability, they accumulate, and once you take the ring off they will all gradually apply over the next few exercise cycles.