Based on the documentation provided, the proper command-line arguments to be used when deploying certificates is CERTFILE=
-UPDATE-
Fixed the problem by manually creating the c:\Program Files\SplunkUniversalForwarder\etc\auth folder and then copying the cert.pem files to it prior to running the install. Then when the install is run, I reference the pem files in the etc\auth folder so that the conf files refer to the correct location. Its still a problem with the installation logic, but a workable enough solution.
-UPDATE-
Fixed the problem by manually creating the c:\Program Files\SplunkUniversalForwarder\etc\auth folder and then copying the cert.pem files to it prior to running the install. Then when the install is run, I reference the pem files in the etc\auth folder so that the conf files refer to the correct location. Its still a problem with the installation logic, but a workable enough solution.