Remap existing attribute at IdP for a particular SP?

[Rob Ansaldo]

We have a commercial SP that insists that our IdP assert an eduPersonPrincipalName for each of our users and that this value be a unique identifier for each user that will not change over time. Our eppn is the user's netid, which can change over time (name changes, class year change, etc). Our employeeNumber attribute does not change over time and we would like to provide this attribute for eppn, but just for this one SP.

I realize that a simple change to attribute-map at the SP would solve this, but the vendor is unwilling to make this modification. I did not choose the vendor and do not have a say in the institution's business relationship with the vendor and have simply been asked to "make it work". Is there any way to accomplish this on the IdP side? We're using 2.3.3.