It's easy to do in Windows RT. You just need compatible drivers.
That's the problem. Microsoft explicitly does not provide support for serial port access in WinRT. (I'm not making this up... it's a well known issue, search the Internet if you don't believe me.)
The reason these vendors supplied serial port access in the first place is because all desktop applications can access the serial port. This reduced the need to use proprietary drivers. Not true anymore.
I guess Microsoft thought hardware vendors would go back and write new driver code for devices they already sold. The reality is that support for older devices is limited to what came with the device. Need a new driver, throw $200 or whatever at a new device (even though your old one works perfectly and will work perfectly for years, and you don't need the new features in the new hardware).
Otherwise, Microsoft assumed that application developers would write the hardware drivers. Frankly, this is absurd, particularly for hardware supplied by third-party vendors.