Changing an object's sortable_title

[Eric Steele]

Hello,

Is there a way I can change what gets returned as the sortable_title 
catalog index value for a custom content type?

I've got Person objects that return a person's full name as the title, 
but I'd like to be able to sort them using a last name, first name format.

For now, I've created a sortableName index for sorting only my Person 
objects, but I'd like to be able to sort them properly alongside other 
content.

Thanks,
Eric