Making agents available only when specifically requested

We have a problem where all of our agents get tied up with long-running builds leaving developers who have a quick build waiting until one of these builds is finished.  Priorities hasn't done much to mitigate this problem.

What I would like to do is have a few agents that won't normally be used for scheduled or vcs-triggered builds, but that developers can select when they manually trigger a build.  Is there any way to do this short of writing a plugin?