Visa does support any Baud rate, the issue though is VISA does not set the serial port speed, your serial driver does. And if I read correctly you are using a non-National Instruments Serial Driver. I would suggest contacting them to see if if this is a known issue with their serial driver.
In the
Knowledge Base you referred to on NI-VISA baud rates it states this:
"NI-VISA itself does not limit the range of baud rates you can use.
Instead, the baud rate request passes directly to the
device driver for
the serial interface. NI-VISA returns a success if the driver supports
the desired baud rate."
-Marshall R