Gavin,
I looked Plex java runtime code where start the initial Applet and found that it only accept two parameter "Function" (full name of function) and "properties" (URL to properties file). That plex generates applet html file as you can see. But nothing like "Parm" or "Parm1" etc.
So it means it doesn't accept parameter at this point. It is kind of amazing no one requested this before.
Now, by looking the code, adding function parameter looks like not that hard.
So I would suggest open issue by pointing this thread. I hope SE team can help for you.
It will be more likely design/spec of how to pass multiple parameter with space char etc..
Regards,
Kiyoshi