If the widget is written in C++ & Qt you can already run it as a standalone app just by executing the binary from the terminal. It will open in a new Window but since qt widgets must have the traslucent background option not to show black background, you will see the terminal app in background aswell.

A bit OT but it may help you: if coded with good manners, widgets will eat cpu mostly if the current desktop is the one it holds them. They keep listening to all desktop swaps but won't start their heavy functions, so you may put a desktop with those heavy ones.
About the memory consumption: you may open a lot of apps/processes to "push" widgets in swap memory, than close all the apps to free ram. It isn't a really clean method, I know. You can check it with conky or the cpu & memory status applet.