I'm no expert on HPGL but as far as I know it's used for vinyl cutters and plotting machines which only have the concept of up or down for the Z (pen) with no movement level. It's either in contact with the material or not.
Some of the Roland Post Procesors appear to be a form of HPGL that have Z levels in them. Might be worth experimenting with those (keep a hand near the stop button!). Dropping a line to email@example.com
(if you haven't already done so) is worth a go as they might know of someone with a similiar machine.