Add queue item to mailkey_alter hook

I'm currently writing a couple of extension modules for subscriptions, and each uses a variable 'field' entry subscriptions.

I'd like to be able to detect the field name as the time of the mailkey_alter hook, so that I can modify the mailkey accordingly. At the moment, I only have the node object available in $arg1 but by adding the full queue item as $arg2, I have access to the field (and potentially other data).

Would you consider this change?