"The ports are still virtualised by the VM hardware in the processor. The task priority process determines which really has it."

Of course - which is precisely what stuffs the dodgy old DOS 16-bit real-mode stuff in NT/XP!

So, if he wants all the advantages of protected-mode multitasking etc he's just going to have to bite the bullet and get a proper protected-mode multitasking-compatible programmer.

Or go multiprocessing and have a separate dedicated PC for DOS only.