Man I'd really prefer it was a crash tested solution in the 3 series like the E46 setup was. I'm not sure PCA will allow that setup BTW. They will not allow 4-point setups, though I did get them to give the E46 coupe quickfit pro an exception because it's DOT approved in that car.
Someone showed up at lonestar PCA last weekend with what look like your setup in an E92, and I didn't call them on it - but technically I should have.
Britax Carseat | Diaper Kit | Redneck Cutouts | Turner Studs | Bassified Individual Audio