BMW rust warranty is for rust perforation of the vehicle body only. I'm not sure if seat is technically covered.
From BMW USA website:
"Rust protection for 2006 and newer models.
For 2006 and newer models, we also provide a 12-year limited warranty rust perforation warranty without mileage limitation. This warranty is for defects in materials or workmanship that results in rust perforation of the vehicle body. This coverage period begins at the start of the New Vehicle/SAV Limited Warranty."