Another way to mimic luck, would be that the spells don't seems to have their intended effect but accident happens making that the target of the spell is getting the same consequence. I think I should give an example, English being not my first language and my explanation sounds... confusing.
The magus cast Pilum of Fire on a target. No fire flies from his hand, however a fire nearby the target burst into flames as some logs mysteriously fall into it, or a flask of alcohol falls in, engulfing the target into fire and dealing the same damage as a Pilum of Fire.
Second example: the same magus cast Hand of Grasping Earth. No hand erupts from the ground however the target feet sink into a hole and stone and roots make him difficult to remove his foot, keeping him immobilised.
Third example: the magus cast Chirurgeon's healing hand on a wound. A candle or a torch falls on the wound as he is finishing is casting, cauterising miraculously the wound without additional pain, in fact even healing the whole thing, just leaving a scarce that will fade quickly over time.
For outsiders, it will look incredible unlikely & lucky and any attempt to replicate will fail miserably (and possibly painfully).
I would see that as both a Flaw and an Advantage:
Flaw (Major - at least): the magus needs a source corresponding to the Form of the spell he is casting nearby his target. Obviously, some spells won't be affected as much or will be either to cast than other. It could make some spells impossible to cast in certain circumstances.
Virtues (Minor ?): Indirect magic. The magic does not seems to come directly from the magus.
Of course, it opens plenty of question regarding certains forms (Vim ?) but it could be a way to make the magus looks very lucky - altough his sodales will know what's up and will probably frown at him for having such an imperfect magic (maybe having the reputation of Dangerous or Unreliable practioneer of the Art).