If the RSQ grille won't fit an S-line, then I'd find it hard to imagine/believe it would fit the "regular" bumper cover either. We in the US don't have an "SE" so I have no idea what that means. From the Audi EKTA, I can see 4 bumper covers : one for the regular line, one for the S-line, one for the India/China regular line, and the RS. For the first three, the grills seem to be the same size and have all the same mounting points. The RS has them in the same places as well, but the RS bumper cover is deeper - like on the RS7 to allow for that enlarged lower vent where they optionally place the "quattro" badge. It's not clear from the illustrations - they're drawings - about the grill itself size, even if the mounting points are the same. So it's conceiveable to me that the grill itself, mounting points notwithstanding could be shorter in height - but reviewing the other bumper covers, leads me to think it (the RS grill) won't fit ANY other cover than the RS.
The other thing I notice from the link above for an RSQ3 grill for $399 - is it is NOT an Audi part at all, but rather a duplicated (counterfeit) piece from China. This is quite common actually and extends to MANY Audi parts - even window regulators, motors etc. So, in their 'creativeness', the manufacturers could've actually resized this part to fit the normal bumper covers - then why it doesn't fit the S-line one is a mystery ...
Since CortinaQ3 has one and by now may have tried to install it, I'm kinda anxious to see what he's found out and if successful ... Hope so to see pics of before/after and during -