I\'m trying to match the contents that belong between a certain ( and its matching ) as found by vim when using the motion %.
You could do this quite easily with a macro. You would search for the keyword with a regexp and then search for the start of the text, then the end of it, then you can undertake what ever action that you want.
E.g. to yank the required text:
qa/somekeyword\s*
/(
lvh%hyq
Now whenever you use the macro with @a you would yank the next bit of text that you are interested in.