Don't use subpage. We are in the process of getting rid of it. When sam marshall made it back around Moodle 2.1 time, it require various core Moodle hacks, which we subsequently tried to get into Moodle core. However, Moodle core eventually want in a different direction with its improved support for what course formats can do.
So, our replacement for mod_subpage is now a custom course format (with an associated module). I believe the new version will not require any core changes. However, I don't know if we will every share all that code. Bits of the course format are rather idiosyncratic (the work great for OU courses, but no-one else would want some of the functionality).
Hopefully, at some point, sam can write a blog about how he built the new version, and someone can rebuild something more generic.