The only logical way to go about this is to make the border purchasable through RP. They wouldn't make it free, as those who bought the new project skins paid the full amount upfront for them, while Yasuo was on sale the first week. But even so, if they chose to make it an addition you could buy, it wouldn't differentiate between those who DID buy yasuo on sale and those who purchased him after. That is exactly one of the reasons Riot won't implement it. The other is that they either A.) blatantly don't give a shit about making a new border and/or B.) that they don't think it would be worth doing.
So, unfortunately, I'm 99.95% sure a project yasuo border will not happen.