I'm looking for an escrow service and/or domain name brokerage that can
facilitate escrow services for the leasing of a third-party domain
name.

For instance, I wanted to lease a domain name for $100/mo from
somebody with an option to buy it out for $5,000 at a time later, and
need someone who can provide this service.

I need preferably more than one service (so I can provide options to
the other party) and that list doesn't include GreatDomains or
Escrow.com, as they've already told me that they don't provide this
service.

Valid answers will include confirmation from the brokerages (either
spelled out on their webpage or via customer service e-mail) that they
provide this service, and payment will come after I confirm this with the service.

Interesting thought.  Leasing a domain name.  What would happen if you
drafted a standard lease/option agreement?

Most of the domain services I use are straight purchase escrow eg sedo.com

The only one I have found (but never used) that does leasing is
http://www.moniker.com/aftermarket/escrow.jsp