It's not really clear what you want to do. But Ctrl+click on PC, Cmd+click on Mac allows you to select or deselect items that aren't next to each other. Shift+click also helps.
Ok, sorry if I was unclear.
I know how to select the subset but I'm having trouble actually performing metadata read on that subset
If I select one line in the Keywording List, I can press the little arrow on the right of that line "Click to show photos containing this keyword". Metadata reads (or any other operation) can then be performed in the lower window.
So, how can I do a similar operation on multiple kewords?
You will need tu use the Library filter for that. So "Click to show photos containing this keyword" reveals the Library Filter.
Then you can Ctrl/Cmd click on another keyword in Library Filter. Also remember it is possible to change the next column in the filter so it is also keywords.