For that we wouldn't really need tight integration - you could conceivably just tell bcache the stripe size of the backing device, and it wouldn't consider partial stripes sequential with full stripes.
But for that to be useful we'd have to have writeback specifically pick bigger sequential chunks of dirty data and skip smaller ones, and I'm not sure how useful that actually is. Right now it just flushes dirty data in sorted order, which makes things really easy and works quite well in practice - in particular for regular hard drives, even if your dirty data isn't purely sequential you're still minimizing seek distance. And if you let gigabytes of dirty data buffer up (via writeback_percent) - the dirty data's going to be about as sequential as it's gonna get.
But yeah, there's all kinds of interesting tricks we could do.