Well I checked into it, and so far, only skills can give you -x percent to resistances.
Deathchill, Study Prey, Flush Out and Trance of Wrath
at Ultimate levels:
I've not done Deathchill yet, but with Study and Trance, it's enough to break Pierce and Physical resistances; while being REALLY close to breaking Elemental resistances: Lightning, Fire, Cold.
With the ability to give granted skills +skill bonuses, hint hint (if possible), it might be possible to be interesting with other characters if study prey was a gskill.
But you're right,
Ronzer's Gift set, Sapros and many many other items only have flat reduce resist.... wait a sec, let me go check again.
EDIT: OMG, The earth shaker has % reduce resists... 30% for one second! So with it, it's possible to break resistances with other support skills, easily.
Cheers for now,
Imp
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) In addition to that, relative reduction can't turn a positive into a negtive value. 
