How can I include a .inc file before hook_node_access runs? I'm currently including a .inc file in hook_init(), but now need to call a function from that include inside of hook_node_access(). I've just discovered that, because hook_node_access() runs very early on, my file is not included. I then tried to do it inside of hook_boot() because the documentation suggests that it runs sooner, but it's still not available to me. I'm temporarily including the file in my .module outside of any function via a simple 

require_once DRUPAL_ROOT . '/sites/all/modules/... 

but would prefer not to do it this way.
First, using the require include to load your file is perfectly sound, I think. Alternatively, you could add the include to the .info file of your module and allow it to be loaded on every page.

However, to answer your question: We could implement hook_hook_info() from a custom module and add node_access as a group for hook_node_access(). Now, in our module itself, we can have our access callback in a mymod.node_access.inc file.

This is probably not recommended as we are dabbling with core (in a small yet unreliable way) with no real value at all but is a way by which you could do it.

Once you have done this, I'd suggest running the entire Drupal core test suite once (at least node module's) to make sure that we are okay.