So after I watched the lolhounds's video on the highest mastery sejuani who is in diamond and plays sejuani in the mid lane I thought I would try it out. I didn't quite have the right rune page or masteries setup, but my gosh I am surprised with the result even though I have only tried it for one match so far. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2356263129/231996695?tab=overview
Why is this downvoted?
You can do it into some matchups and not others.
It used to be useable in a wider range of matchups when her % hp damage was on her charge so you max q first and beast mode the enemy laner easy.
Janna AP mid :)
Is it a thing in the meta? No. Could it work? Sure.
Is it meta? No. Is it a thing? Yes.
sej is a bad laner and a good ganker, why would you pick her mid
Match number 2 results were kind of sad. The enemy gave up like half way into the match :( I feel sorry now. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2356265439/231996695?tab=overview
She's much better top than mid. Just farm and wait for the jungler or enemy bot lane to be over extended so you can tp gank. 
master race xD
i use 
top as a pocket pick sometimes, and honestly feel that mid sej can be good, her ultimate is a godsend always and can build very tanky to deny other mid laners poke and overall burst, like the video shows, it involves the jungler abusing the mid laner early due to sej's VERY bad early waveclear (ie can't last hit fighter minions after 2 turret shots full hp level bad) until she gets 
then her clear is over edge good. and getting her fed.
cheeky %max hp on w that feels fun and fair in most respects.
