Hi Andrew
No, I don't have Parallels Pro, but the boot selection stopped working in Parallels 10, so I think it's not associated with the Pro version. Besides, the option is still available in my version (Hardware->Boot Order-> Select boot device on startup).
I got around to trying VirtualBox. Setup is quite complicated, but Mac OS X 10.5 can be made to work. I have used both a machine which was capable of running it directly (iMac 24 inch, Early 2008) and a newer machine (Mac mini, Late 2014). I think performance is as good as Parallels, at least for what I want to do. Some things are missing (e.g. clipboard sharing and disk sharing) but I can work around those.
Let me know (off list) if you are interested in how I made it work. My address is in the conversation.
William
Last edited: Oct 10, 2015