I'm not sure how the current vm system tracks dirty pages, so I wouldn't know where to start suggesting realistic solutions. But yes, assuming the dirty pages are organized in some sort of specialized data structure (queue or otherwise), you could do this without needing new barriers: it would suffice to create a new dirty-page root structure before starting writeout, and redirect freshly dirtied pages to the new root. The writer process can then take its leisurely time committing the pending writes to disk without obstructing other processes issuing new writes. New writes to pending pages can be reparented to the new root via an RCU mechanism (to not violate previous write barriers), and incomplete writes can be reparented by the writer process in a similar way. When the writeout process completes, the old root can be completely discarded; the writer could even consume and free the old structure while traversing it.

But really, this is all fantasy given that I know nothing about the virtual memory internal data structures.