with respect to a public employer that has a reservation of rights clause relating to welfare benefits, do the state and federal constitutions' prohibiton on impairment of contracts play a role?

caselaw searches return nothing.

I'm not sure what you mean by reservation of rights. The discussion in Strunk v. Public Employees Retirement Board, 338 Or 145, 108 P3d 1058 (2005), about the folllowing Oregon Revised Statutes 238.375(3) might help:

"No member of the system or beneficiary of a member of the system shall acquire a right, contractual or otherwise, to the increased benefits provided by [ORS 238.375 to 238.380]."