I had a sneaking suspicion that there was a reason the expression ‘seeing as though’ had been “hitherto unstudied”. And that reason was… (drumroll)…
Nobody actually uses it.
The phrase seemed (and still seems) more like a typo or a slip of the tongue to me.
However, I decided to do a search of COCA and the BNC anyway. I got 3 hits in COCA (all in works of fiction) and none in the BNC. Then I did a search of the NY Times and got 8 hits. I was quite unprepared for that! So, it seems that at least a few people actually use ‘seeing as though’ (gasp!), but I can’t say that I’ve ever noticed anyone I know using it.
Here’s a sample from the NY Times, apparently written by a teenager who was posting an opinion about an article that had been published:
“Seeing as though I want to go to college that requires me to take the SAT’s and most likely the ACT’s which can be very stressful on a young student.”