I have a TreeDropdownField working exactly as I need it to, however I'd ideally like to disable checkboxes for all pages in the tree except for those which have the ServicePage class.

Here is my code:

$fields->addFieldToTab(
    'Root.Services',
    new TreeMultiselectField(
        'Services',
        'Related services',
        'SiteTree'
    )
);

Looked at the code suggested at the bottom of http://stackoverflow.com/questions/29171542/filter-sitetree-treedropdownfield-in-silverstripe-3-1 but not quite sure how to integrate this into the above. Can someone please advise.