I realize this is several months late, but ...
I have to deal with .plt (hp/gl) files regularly in my Mac environment. It was a real challenge for a while, as Mac guys were generally unfamiliar with them, and even software companies that dealt with .plt files for their Windows versions ignored them on the Mac.
Here's what I use to 'crack the case':
(i) CorelDraw - has fairly decent HPGL import/export filters, and version 10 runs on OX X Tiger. You can probably find it on eBay cheap;
(ii) Cadintosh - a $30 shareware program that allows HPGL import and export, to other formats, of course; and,
(iii) My new favorite -- the Unix command line program HP2XX, which can be installed via Fink and run from the Terminal. It converts HP/GL files (.plt) to a myriad of formats, including DXF.
I hope this helps. Frankly, since TurboCAD for Windows handles this format, you would think the Mac version would too. But, even Adobe Illustrator only supports .plt files in the Windows version, not the Mac.
The puzzling part of this whole thing is this -- HP/GL is not that complicated a language. I had an Apple II program (RoboCAD) that generated these files with less memory than a musical greeting card has today.