The 755CM is great machine. My 755 works since 6 years without a single problem. It is in use every day and has produced thousands of drawings.
Since the 755CM is a postscript printer it does not need a special driver in Classic MacOS. It only needs a ppd file which tells the apple laserwriter driver which capabilities the printer has. (page sizes, color, etc.) I think the same is true for OSX.
HP does not translate this simple little file to OSX for marketing reasons, in order to get us to buy new plotters.
However HP is not a software company. It has produced the worst software I have ever seen. My OfficeJet G85 is almost unusable because of the badly written HP drivers.
Missing printer support for the 755CM is one of the reasons I did not yet upgrade to OSX, but I am very interested to solve this problem. In my experience most printers are not very picky in regard to the ppd files used.
Perhaps you could try to use the Designjet 800 ppd or you could use the Classic OS 755 ppd. Good luck!