An RS-232 to USB converter won't work with the DMM. Normally those converters connect the USB side to the controller while the RS-232 connnects to the device. For those that need RS-232, it is usually the controller that has RS-232; while the device (DMM) is USB. This is difficult for converters since they need to have the proper USB drivers to talk to the DMM. There aren't many (if any) of those out there.
A better method is to use RS-232 to Ethernet converters. I did a write up on a blog for this: http://gpete-neil.blogspot.com/2013/07/rs232-to-control-lxi-instruments.html
I'm not sure that I understand how a RS-232/USB converter could be used in that case. RS-232 can only talk to one device at a time. How would a system with multiple IO's use the converter? If they have USB, wouldn't they just use USB? Not trying to be snarky, I'd really like to understand how to get this to work.
If the instrument and the controller both have USB, of course that's what you'd use. But, older instruments such as the 34401A were designed before USB and it was never added. Tha started with the 34405A/34410A/34411A series a few years ago.
The devices that you listed allows the controller to use RS-232 via the PCI or PCIE bus. This still does not solve the problem of how to to get RS-232 to converse with a USB device. My point is that the converter needs to have the right USB protocols in order to talk to the DMM on the device side. RS-232/USB converters work because they communicate easily to the USB controller since the drivers work with any PC. However, on the device side you need unique drivers for every type of USB device.
The suggestion in your blog post was to use a RS-232/USB converter to talk to the new 34461A which I don't believe is possible. However, a RS-232/USB converter would work quite well with the 34401A.
Perhaps we can take this offline, once we come to an agreement we can update the comments section.