I've updated the MIME Tools plugin to add support for SAML decoding. The patch is available here:
The reason that the MIME Tools plugin was updated and new plugin wasn't created is because decoding SAML messages requires URL decoding and BASE64 decoding, both of which exist in the MIME Tools plugin already.
It may be buggy so please submit any quality reports.
Updated binary attached.
Warning: It's not a great idea to download binaries from forum posts (for security reasons) but if you can't compile it from source then use this at your own risk.