You can draw your KEGG pathways as SVG graphics. Then your pathways will become interactives
You will use the png file of the pathways as the background and in the foreground you will draw your genes as SVG objects. Each object will have an id that will be the Entrez Geneid of the Gene.
You can place the SVG objects at the right place thanks to the coordinates that are available in the KGML files.
Let's say that you have in the backend a database with the Entrez Geneid that are linked to Enzyme classification or whatever data then you will be able to hihlight the object (genes) that match a given criteria. I personnally use Ajax techniques to do it dynamically. See below two instances
The first one highlights ATPases in Cancer pathways:

The second one highlights Proteases in Cancer pathways:
