Does Namely have the capability to use a previous or future-dated salary for life/voluntary benefits?

No.  An employee's current salary is always the basis for life/voluntary benefits.

Namely cannot base life/voluntary benefits on a previous (and currently end-dated) or future-dated salary.  

Note: If an employee's salary changes, and the carrier allows for a re-calculation of life/voluntary benefits based on that change, the client admin can use the Enrollment Wizard to update these benefits via an Administrator Change life event.