Legalisms aside, why would you want to bless yourself with your left hand, given the connotations I pointed out earlier?
I can think of one reason only: The Church Of Me.
Also: I have seen unusual demonstrations of piety at the Latin Mass I go to. They tend not to persist. I’ve drawn conclusions from that.
Perhaps that is because you think with tunnel vision; you presume that because someone may do somthing differently, that they do it in some sort of spite attitude towards the Church.
There are extremely few people this applies to, to begin with, because the vast majority are right-handed. And since I presume you are right-handed (and I do so because you appear to have no understanding of what it is like to be a left-handed person in a right-handed world), it is easy to presume that what you do with ease and naturally, absolutely everyone else does likewise.
Try this, just for giggles. Next time you are eating something which is small - say corn or peas, and with a fork - try eating them with your left hand. Maybe the experience of trying to do something that is going to be completely awkward will give you enough of a jolt physically to understand that there are a few people out there, very few, but a few for whom making the Sign of the Cross is going to come naturally with the left hand instead of the right hand, and using the right hand will be extremely awkward.
I would presume that you would have enough grace, let alone common sense, to understand that physical limitations may be a bit more than you have experienced, and possbily, just possibly, doing something differently has nothing to do with an attitude of rebellion, but is simply a matter of physical dexterity or lack thereof.
Would you criticize an elderly priest who has knee problems for not genuflecting at the Cosnecration? Is he too a member of your identified group of “The Church of Me”? People with arhtritis may be able to genuflect, especilly if they have help getting back up. Are you going to insist the priest with arthritis genuflect?
And no, being very dominant left-handed is not the same as arthritis; it is just a matter of extreme awkwardness.
This isn’t an issue of a whole bunch of people starting to make the Sign of the Cross left-handed. It is about a rare individual who has a significant problem doing so with any sense of coordination. The Church of Me? No, I don’t think so. Try eating left-handed and see how awkward that is. Maybe you will get it.
Then again, maybe you won’t.
There are none so blind as those who won’t see.