Use Privatemsg admin form in theme (Missing functions!)

Question

I've exposed the privatemsg_list_view() form in my site theme with a bit of work and it's displaying well. However, updating message statuses or trying to delete bulk messages results in a sad error:

An AJAX HTTP error occurred. 
HTTP Result Code: 200
Debugging information follows.
Path: /system/ajax
StatusText: OK
ResponseText: Fatal error: Call to undefined function privatemsg_list_submit() in [...]/includes/form.inc on line 1464

Now I know that privatemsg_list_submit() is indeed set as the form #submit handler and I know that it does exist, over in modules/privatemsg/privatemsg.pages.inc. Obviously there is a disjoint between /system/ajax and my template.

What needs to happen to make this work? Do I just have to attach the .inc file to the form handler somehow? I've tried playing with module_load_include() to no avail.

Suggestions?

Update: The only place I have tried adding the module_load_include() function is directly before the function call in /includes/form.inc - that would have been somewhere just before line 1464. Obviously that was just a hack for troubleshooting purposes... should the module_load_include() function be placed somewhere else?

For reference: Call to undefined function privatemsg_list_submit()

Update 2: I have also tried inserting module_load_include() in a custom module_init() function as well as inside the theme hook_form_alter() - no luck.

Update 3: Manually including the file within a module_init() hook makes it work. Here's what it looks like:

function MYMODULE_init(){
    if($_GET['q'] == 'system/ajax'){        
        if ( preg_match('/privatemsg_list/', $_POST['form_id'])) {
            include('/great/big/system/path/web/sites/all/modules/privatemsg/privatemsg.pages.inc');
        }
    }
} 

While this appears to work on the surface, I'm sure it's not the right method. Is there a better function hook to use to include this file?

Answer 1

Though I suspect that this is an improper workaround, the solution was to hook a custom module's hook_init() function, watch for a system/ajax path and include the file if needed:

function MYMODULE_init(){
    if($_GET['q'] == 'system/ajax'){        
        if ( preg_match('/privatemsg_list/', $_POST['form_id'])) {
            include('/great/big/system/path/web/sites/all/modules/privatemsg/privatemsg.pages.inc');
        }
    }
} 

UPDATE: Answer is available at https://drupal.org/node/1961776! In short: use form_load_include() instead of module_load_include().