Hi Benjamin,
if you don't plan to support multiple devices per account just the t_suspend/t_continue pair should be enough to support push notification.
This being said you can easily integrate the example in my presentation in the Kamailio's configuration given in the Asterisk/Kamailio integration page.
For the INVITE you have to modify the route[LOCATION] and, instead of performing the lookup, suspend the transaction and send the push. In this way you'll also send the push for every INVITE, regardless of the registration state. You have then to modify the REGISTRAR route after calling save() to resume the transaction or to append new branches (if you use tsilo). I think that the snippets from my example should be enough.
Hope this helps.
Regards,
Federico