GTD/UTD takes over where any of those finite-element type method falls apart. Uses canonical shapes. ALDAS is one example. Often used for V/UHF antennas on large airplanes.

Google GTD UTD and ALDAS to start.

Not sure how well it will work for non-metallic humans. MIght be able to set upper and lower bounds though...

I don't think you'll get a time domain code for that size structure. Wire codes only, and accuracy is so-so if you are looking for fine structure, although 2.5 inch wavelength antennas with 400 inch structures usually have to live with multipath problems.
It sounds like antennas on aircraft.
If you're having reflection problems off structures causing ripples or phase uglies in an interferometer array, my company has some interesting thin peel and stick absorber products that absorb rf energy at veriable angles all the way up to grazing incidences.
kchiggins
Toyon Research Corporation.

The same company that makes HFSS (Ansoft) makes another product called Maxwell 3D.  It's mainly for modeling magnetic fields but maybe you could use it somehow on your application.

Higgler - I'm working with Toyon right now on a set of GPS antennas for the GIF fuze.  Nice to meet you.