I have a Canon i560 ink jet printer (parallel port) that I want to output from a piece of test equipment that has a DB9 serial port that uses either the Postscript or Epson printer protocols.

Other than the correct cable, would/could this work?

Well, the i560 is listed as "Partially" on the OpenPrinting database...basically that means the printer will work in a very limited fashion. It isn't a PostScript printer and I'm not sure whether or not it adheres to Epson protocols, either...

So, if it is nearly impossible to work under Linux, then I would say, from a piece of test gear that only uses PS or Epson...not bloody likely.

The design of the device is around 10 years old.

Always worth a try if you don't have to invest money in the attempt.
I used an old HP ink jet to print from a line monitor recorder, it did work.
Back in those days you had to print directly from the device output port, these days the monitors allow the file to be downloaded to a PC.

The problem with many inkjet printers is that very little of the printer is devoted to anything other than putting ink on paper. Some companies have more electronics devoted to checking and monitoring the ink levels/cartridges in their printers than they do for any print 'functions'. All the functionality is dependent on the Windows drivers.

Want to print a PS document...the driver first reads it, then 99.9% of the time translates it into some half-baked, proprietary format that only that company uses...then it sends it off to the printer (or, more likely the spool, where it may/may not be translated a second/third time).

Things have changed over the past couple of years, though...a large number of recent inkjets are less dependent on Windows and have moved more/most of their functionality back to hardware resident on the printer.

Why?

Mac OSX and to some extent the CUPS/Foomatic developers...

Most HP inkjets understand PS...most, except the newest, Canons...don't. (Canon was one of the companies that for a a long time swore that the would never support *Nix for their inkjets and never would depart from their proprietary formats...)