Here is what you can try to do is to move task details page to be under OOTB My Inbox page. This way it will inherit access privilege permissions from My Inbox page and you don’t worry about access privileges on the custom task pages and may just well not grant them to hide.
To accomplish it you need to modify in your application /WEB-INF/tasks//taskDetails.xml file and change top level element which defines where page is deployed:
#webMethods#webMethods-BPMS#MWS-CAF-Task-Engine