Two answers here - almost all our pre-defined alerts use custom alert templates for the alert body - but not the subject. The subject of the alerts just inherits the default alert template subject lines.
So you could add the new property in the default subject template.
The other option is to set the alerts to be delivered to a user by the user.pager address, and select that user to receive alerts via the sms template. (The sms address of the user can of course be any email address - not necessarily an SMS address.) Selecting the pager delivery mechanism with the sms template makes all alerts use the default SMS template, regardless of whether there is a custom alert message - and you can then change the default SMS format to whatever you want.
If neither of those approaches work, let us know and we can probably work out an automated way to change all the custom alert messages in your account.
