On a normal keyboard, left and right modifiers are necessary since they're designed that you only use each hand for keys on each side. You need to move one hand to reach the modifier, so the idea is that you use the alternate modifier to the hand you need to press for the key, if you see what I mean.
That said, I never trained as a typist. so I use all fingers for all keys, and judging by wear on most of my keyboards, I use the larger left shift almost exclusively.
On the Pandora, since you use thumbs for keys and fingers for the modifier shoulders, I have no problem reaching, say capital A using left index finger and left thumb. Or capital L using left index finger and right thumb. Perhaps that's influenced by my general keyboard ambidextrousness, but I don't see the need for a right hand shift shoulder button.
More power to you if you want to mod your Pyra though, I'm just less convinced that your proposed mod should be designed in.