Is there anyway to "get credit" for post Masters work on a resume?
Research assistant is a job as well as part of an education, and it is common to see it listed as part of one's appointments on an academic CV.  After all, you are being paid for the work you do (even if typically at a subsistence level).

It would thus seem quite reasonable to list it that way on an industrial resume as well, as part of your list of prior employment, thereby providing a clear statement of your time working in that capacity.  Whether your HR department is reasonable enough to accept this or not is an entirely unrelated question, I'm afraid.