By coincidence, one of the tutorial examples for the MultiNEC program
deals with the subject of loading coils on a shorty dipole. With
MultiNEC you can define the antenna model using variables for both the
amount of reactance in the coils and for the placement of the coils
along the wire. Each time you change the variable that controls the
placement of the coil you can use the MultiNEC "Resonate" function
which will automatically invoke EZNEC in an iterative manner and
adjust the reactance of the coils until the antenna is resonant.

The example was written to illustrate and explain various capabilities
of MultiNEC, of course, and not to address the particular scenario of
this thread. It's available in standalone form at
www.qsl.net/ac6la/mnex3.html.

Dan