no doubt that treadmill motors are great, but not for bicycle conversion, they are mostly used with trikes.
i did thought about it but to mount it on to bicycle it would be a great challenge, and the end result would be ugly e-bike. you would have to both bolt and weld support for it on to the bike unless you have steel bike then you can just bolt it. also it requires longer chain setup which is not very good idea for e-bike conversion, your chain needs to be closest to hub sprocket or driven sprocket as possible.
the e-bike motor needs to be light and efficient, think battery weight. less weight on the bike more batteries you can add on it. also e-bike needs to look nice, not like some i have seen with treadmill motors (junk yard class i'd say) i wouldn't sit on it even if someone paid me
my bike if i ever complete it, looks good as it is, currently i'm building the mid frame battery box which i will be able to remove and bring it in with me for charging, kind of convenient.
always think of this when you are make something; creation is mirror