I am not sure whether you are talking about a preview function or a remote monitoring option.
To my mind a "preview" funtion implies that you are seing here what you will go to the output when the next cue is executed. Therefore it is a seperate output. This is how it works on vision mixers anyway.
The remote desktop option discussed here would only be a monitor option. It merely shows you what is going out already.
I have tried all sorts of VNC options and they all put strain on the processor (because the client runs in the background all the time).
The easiest and most reliable solution to avoid this is a Simple VGA DA. If you do not like to run thick expensive cables you can use VGA to CAT5/6 converter boxes. If you want keyboard and mouse control you could use USB extensions or a KVM extender. If video suffices you can use a scanconverter as well.
Since the monitor option is not a big problem if you have the cash for a few extra boxes, I am very interested in a preview function - even at Pal resolution and DVPal codec: Maybe this is possible via the composite out on the capture card (depending on model)?

Best wishes,

Sven