I'm not an SME here but I did find this link in our
TechLibrary: http://www.juniper.net/documentation/en_US/junos12.3x48/topics/concept/gprs-sctp-configuration-overview.html -- perhaps this will get you pointed in the right direction -- the supported platforms for SCTP are on the right.
This link talks about NAT on MX: https://www.juniper.net/techpubs/en_US/junos14.1/topics/example/nat-inline.html
These links do talk about different versions, not sure what version or versions of Juniper products you are on.
I hope this helps.
Cordelia