Firstly thanks to andrey_gta for starting this thread/adventure and all who have contributed along the way!
I think I may have found a possible alternative blank rotor for the 135i available in the US. Thanks to brembo/zimmermann for their rotor drawings I was able to come up with something that seems to make sense at least initially.
Given that a 330mm x 20mm seems to work (F10 rotor, e90post thread); I've come across a 330x20 rotor from the 2012 335i that is 330x20x66mm (11mm shorter than stock).
It also doesn't seem to have that flange/shoulder that the F10 rotor has (that required modification to fit with the dust shield).
Its BMW part no. 3421686489, Zimmermann part no. 150290420.
Stock 135i rear:
Hub face to rotor/caliper centerline: 59mm
2012 335i Rear
Hub face to rotor/caliper centerline: 50mm
I really haven't had time to dig into this too much as of yet, but figured I'd throw it out there for those who might know a bit more about it than me (and check if I'm missing something). If it seems like it would be viable I'd be willing to give it a shot.
FCP link to the rotor ($59/piece):
https://www.fcpeuro.com/products/bmw...99#description