Unless you accept the loss of an i, the best such word is pity.
That’s because in that case Latin pietās, pietātem was whittled
away till it had no antepenult left to it, and so you have no chance
to stress something that isn’t there. :) The version that didn’t get
quite so pared down became piety, which is stressed
antepenultimately like most of the rest of them.
Even very short words of this pattern have the stress on the antepenult, like acuity, amity, deity, fatuity, laity, paucity, unity. The most common of these words end in ‑acity, ‑ality, ‑anity, ‑arity,
‑ariety, ‑bility, ‑eity, ‑idity, ‑ility, ‑inity, ‑iety, ‑ivity,
‑ocity, ‑osity, ‑uity. Some are even productive as Modern English suffixes in their own
right, like ‑bility. And all are stressed antepenultimately.
These words all come originally from Latin ‑tās, ‑tātem for a
state or condition. All the rest either retained enough syllables
to retain an earlier stress, or else had the intervening i lost
altogether, as in safety and plenty. (But compare royalty).
A few such words had the Latin suffix joined to a consonant stem
as in faculty, honesty, puberty, but here again you have no i
before the ‑ty — and you still have antepenultimate stress.
To have another syllable stressed than the antepenult, it would
need either a “long” i sound (actually a diphthong) in the penult like whitey or almighty, or be
stressed at the end, something which doesn’t really fit the English
stress pattern but can be found in the common pronunciations of
loanwords like actualité and vérité from French.
There is, I suppose, still the possibility of preäntepenultimate
stress. The closest match there is difficulty /ˈdɪfɪkəltɪ/ from
Latin difficultās, ‑tātem, which may have gone through a French
difficulté en route.
The same process that yielded pity also produced city from Latin cīvitās, ‑tātem via French cité, but that is no longer an abstract noun.
The OED has one other candidate that meets your criteria, but it is obsolete now and was never more than rare (and was, perhaps, catachrestic): succity, about which they say this:
† sucˈcity. Obs. rare-1.
Etymology: f. L. succus juice, sap + -ity. But ? an error for succosity.
- 1646 Sir T. Browne Pseud. Ep. ɪɪ. i. 42 ― A lapidifical succity, and principle which determins prepared materials unto specificall concretions.
How they inferred that the stress was on the penult, I do not know.
So yes, it’s a pretty foolproof rule.