SP: extract LHS of scoped value

[Tom Poage]

My Google-fu is failing me. Is it possible in the SP to extract/use the LHS/local part of a scoped value e.g. ePPN so it's available to mapped into REMOTE_USER?
 
I (believe I) understand ApplicationDefaults (cf. REMOTE_USER), and have (re)read NativeSPAttributeDecoder and kin--even a small amount of the SP source--but am just not finding/getting it.

We have a new SP installed with specific requirements for a *NIX-like loginid (no '@' etc). I'm trying to be lazy and avoid wiring up a one-off attribute in the IdP to meet this SP's requirements (and our 'uid' is multi-valued, so not so good as an identifier).

Thanks.
Tom.