i ran across this earlier today, i think it might be a way to provide the basic functionality in this RFE using OOTB picketlink. Basically, configure each SP to use a different ACS, each protected by a different auth-constraint (role).

https://issues.jboss.org/browse/PLINK2-87 (see the comment from pedro)
https://github.com/picketlink2/picketlink-quickstarts/tree/master/saml/employee-multi-protected