hook_civicrm_membershipTypeValues() not always called

I'm trying to get the cividiscount extension working and I'm having a problem with getting access to pricing for membership purchases. We have overridden hook_civicrm_membershipTypeValues() to modify the price and label of memberships when a discount code is entered. Unfortunately, the hook isn't always called when a membership is purchased.

Using the main contribution form 'Membership and Renewal' in a standard test site calls the hook, with a flow something like:

civicrm/contribute/transact?reset=1&id=2 ->
CRM_Event_Form_Registration_Register ->
CRM_Member_BAO_Membership::buildMembershipTypeValues() ->
CRM_Utils_Hook::membershipTypeValues()

But when I submit a credit card membership using the contact tab form, the hook isn't called. The data flow is something like:

civicrm/contact/view [submit credit card membership] ->
civicrm/contact/view/membership?reset=1&action=add&cid=69&context=membership&mode=live
CRM_Member_Form_Membership ->
buildCustomData() [ajax call]->
civicrm/ajax/memType ->
CRM_Member_Page_AJAX::getMemberTypeDefaults()

Unfortunately this last call directly injects the price onto the form with no hook called.