Could you use ipset type "hash:net,port". Then use
"0.0.0.0/8" in place
of the "net" ? Not sure if this will work as expected.
Did you mean to suggest using 0.0.0.0/0 (you wrote 0.0.0.0/8)? If you did, that will not
work as a work around as ipset type hash:net,port does not allow zero prefix size entries
for "net".