If you expect various parts output of page to Q2A core, it is wise to use page plugin. If you dislike page plugin when you make PHP page, I think that it is necessary to remodel Q2A admin -> page feature. For example, $custompage '[content]' is analyzed by eval() in lower program.
qa-include/qa-page-default.php (about L75? [case V1.6])
Because this is not comply with Q2A manners, you must carry out this method with your risk. I hope that Q2A page feature will support PHP in the future. However, it isn't supported yet. This resembles the flow that WordPress strengthened static page little by little.