How do you get HOMER to model a medium penetration system, one where you have to have a diesel running at all times, whether or not the capacity is needed, and it must be running at least at it's minimum allowable load?

Yes, you can force the system to operate at least one generator by setting the operating reserve as a function of wind power output to 100%.  That's the last input on the Constraints window.  By setting that parameter to 100%, you're saying that every kW of wind power output requires one kW of spinning reserve to back it up, so the system can never operate on wind power only.

Another possibility is to force the generator to operate at all times using the generator schedule.