You can separate transmit and receive IPs, if your tx network does not implement source filtering.
So you can tx from one place and receive from one or more other places "sensors" that you use to receive SYN-ACKs.
You can use several (an arbitrary number) of spoofed source IPs on tx to hide your "real" rx IPs, at the cost of more egress traffic.
There is a technique involving ipids (idle scanning) you can use which does not reveal your IP at all but it is not reliable; read: not usable beyond very tiny scale. You could put a lot of effort into it but it's not worth it. Nobody beyond a few vociferous cranks _really_ cares about IP scanning.
The real way to stay off radars (eg dshield) while mass scanning is have a ton of unrelated IPs and scan as slow as you can stand. This assumes good randomization (not obviously striping across networks from the same IP).
