I'm not sure why here iomap was excluded technically or I'm missing some previous comments? Could you also make iomap support new FIEMAP physical extent information? since compressed EROFS uses iomap FIEMAP interface to report compressed extents ("z_erofs_iomap_report_ops") but there is no way to return correct compressed lengths, that is unexpected.
I'll add iomap support in v4, I'd skipped it since I was worried it'd be an expansive additional part not necessary initially. Thank you for noting it!
Sweet Tea