I think this would be very difficult: the longer North China is lost, the harder the reconquest is. Liu Yu's accession as Emperor helped the wind out of the Eastern Jin's reunification sail, and his dynasty had a considerably unstable government. Unification is possible, because the Liu Song did possess large parts of Northern China.
I think your best hope would be to have the Liu Song state prevent the Northern Wei from taking all of Northern China, and then creating a completely ATL figure, born after the establishment of the Liu Song. This is because the figures around at the establishment of the dynasty are decently capable but not enough to handle a task as momentous as reunifying China.