Finding the total number of papers published in a given journal, for a given year Is there any way that one can find the percentage of papers from a certain journal that meet a given keyword? 

The numerator of that fraction (the number of papers that conform to the keyword who were published in a given journal) can be found easily enough with a simple search. But the hardest part really comes down to finding the denominator of the fraction, that is, the total number of papers that were published in a given year, across multiple journals (thus, using the same metrics for all to enable comparison).

Services from Web of Science/Knowledge (InCites JCR etc) don't seem to help here. The only resource I found that gives this total number is https://scirev.org/, but many important multidisciplinary journals or general-psychology journals are not listed there.
If the journal is indexed in Web of Science, this can be easily searched for. You mention it doesn't seem to help, but I don't understand why. In the main search window, search for 1) publication name = [name of journal] and 2) published in whichever year you're interested in.