Quickie alias/bind to toggle the HUD Target display between box and 3-D. This comes in handy for distant targets as the 3-D boxes can be nigh invisible from afar. Copy and paste the below into your wgaf.cfg file. If desired, bind HudBox to a key of your choice by replacing F6. Sorry it's not one of those fancy plugins but it's only four lines.

alias HudBox "HudBox1"
alias HudBox1 "set rTargetBoxMode '1'; alias HudBox 'HudBox2'; echo '>>> 3-D <<<'"
alias HudBox2 "set rTargetBoxMode '0'; alias HudBox 'HudBox1'; echo '>>> Box <<<'"
bind F6 HudBox

Would it be possible to have it automatically switch based on target range?

Yes, you could make a Lua script to do that. I might do it if I'm bored. :)

Toggling manually would still be useful. For example, when targeting turrets on cap ships; much easier to locate using boxes.

If switching automatically from box to 3-D, could that be keyed to when the target gets close enough to start enlarging the box on the HUD (and vice versa)? That would make it resolution dependent rather than on distance since a small object would need to be closer than a large one for the 3-D box to be useful.

We currently can't detect the actual size of the box, AFAIK.