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.