I think user Nickmattocks has the Bean and Shortboard so he may chip in at some point.

Otherwise: this is from the FAQ with regard to shortboard footswitch assignments http://line6.com/support/docs/DOC-2301

Q:  How do I assign an effect or amp block to be toggled with a foot switch of an FBV controller?

A:  In the Signal Flow screen, select the FX or Amp Block which you want assigned to one of your footswitches. Then Press and hold the ENTER button to display the Footswitch Assign screen. The Footswitch Assign page is display is similar to your Signal Flow view. Use Multi-function Knob 1 to select the footswitch to which you want this FX Block assigned.  You can also just press the FS switch you would like to assign it to that amp or effect. NOTE: you can assign multiple FX Blocks to a single footswitch or Exp Pedal's "toe switch". You can reference the current Preset’s FX footswitch assignments by pressing the VIEW button to display the Performance View screen.

This doesn't quite cover what you are looking for but suggests to me that assignment is the same as with the HD500 built in controls.

In edit then, you should be able to select an amp then go  to the controllers screen and assign EXP1 to the volume.  In the pic below this is what I have done for the 'Bomber' amp.  If EXP1 does not work, try EXP2

Click on the pic for a less fuzzy view.  You can get the same assignment options up on the HD500 by double-pressing the 'Move' key.