I can't say I'd ever see SE being okay with a perpetually renewable Stoneskin without giving it a worthlessly low cap or limiting the conversion rate from Enspell damage. In general, they've seemingly been a bit fickle with Stoneskingas in Earthen Ward and Diamondhide being meh in the player's hands. Accession and Stoneskin is a later exception to this, but one might argue the stratagem cost and increased MP/recast was also part of their balancing. The fact items that enhance it don't carry over to the non-caster also seems like a nod toward their dislike of physical invulnerability that isn't a 2 hour or limited to a rare temp consumable.

You're also asking for a bit of a code nightmare since not only will the mob need to store enspell damage done, but then document how it's been applied to other players. One-handers will strike more often, thus you could see them getting 20 here, 20 there, etc.. Two-handers, however, might wind up getting 40 per whack. Without making this consideration, you wind up making the ability better for one-handers, and I'm not really game for prompting segregation amongst the classes.

The above is also why I didn't include Stoneskin, Blink, or Aquaveil in my talk about a perpetuating aura, as these are not fixed effects and vary as you take damage or absorb a hit. I also realize I forgot to include Regen in the category of buffs that would work.

More toward Enspells in general and popular ideas I've seen over time, giving them additional enfeebles is not something I'm totally against, but at times it seems redundant, worthless, or too good NOT to use like if applying Stun to Enthunder, thus making every other Enspell choice a bad one. I've been leaning more to the pool/level/sublimation mechanic ever since those and WHM's Solace came around, but honestly, how SE would decide to convert it is up in the air. I know with my old Elemental Feedback idea, the offensive side was more like a Saboteur on a 10s timer for debuffs (though it favored durations more than potency). The defensive was more like SCH's Perpetuance stratagem, though also offering potency boosts.

...which is all reminding me of how ideas for RDM have come and gone, often finding their ways too other jobs. Bleh.