This article clarifies whether you can configure NAT rules on traffic by using policy-based VPNs on SRX devices.
NAT is not working for policy-based VPN traffic on SRX and J-Series gateways. NAT rules are not getting applied. This applies to Junos OS versions 8.5 and later.
Currently Junos OS does not support NAT for traffic with policy-based VPNs. Presently NAT can only be configured for route-based VPNs. Check Route Based VPN V/S Policy Based VPN for additional comparison between route-based and policy-based VPN.
2020-10-13: Article reviewed for accuracy; link updated; no other changes made