You should be able to!
In include/class.ticket.php, around line 488, is
function replaceTemplateVars($text){
This is the function used to replace the variable placeholders in the email templates with their real value (ie it turns %subject into "My Ticket Subject").
There are two things to change. You need to find $search = array('/%id/',
Before the '/%id' insert your placeholder, so it may look like this:
$search = array('/%time_spent/', '/%id/',
Next you need to find $replace = array($this->getId(),
$this->getExtId(),
and reference the function to return the number of hours:
$replace = array($time_spent, $this->getId(),
$this->getExtId(),
I have used this before with other variables, so it should work. It would be a lot neater if there was a get_time_spent() function, but that would take a fair bit more editting, and this should get the job done.