Having the netscreen in front makes it more complicated and is probably not needed if you are leveraging an SRX550.
I did something very similar with a 550 a few years ago. Basically I had a few devices that were replacing ISDN for radio broadcasts. They were on their own VLAN with private IPs but needed to be publicly accessible and wide open with no ports blocked or filtered. Here is what I did:
set security nat static rule-set ISDN-mock from zone untrust
set security nat static rule-set ISDN-mock rule device1 match destination address 88.8.8.8/32 <-- your outside IP
set security nat static rule-set ISDN-mock rule device1 match destination-port 0 to 65535 <-- all ports
set security nat static rule-set ISDN-mock rule device1 then static-nat prefix 10.15.16.22/32 <-- private IP of your device
Then setup a standard security policy so incoming traffic coming through the NAT above is permitted.
When using static NAT, the source IP when going outbound will be the 88.8.8.8 address. So the source and destination NAT addresses match.
It's hard to say what will be needed on the NS. How do you have the configured to work with your SRX?