Calculate sum in View of ISO-style timestamps
Imho your two best bets would be to either create a custom formatter, or to use hook_field_attach_view_alter() to modify the display (this answer might help if you go down that road). I'd be happy to expand on either of those methods.

Is there any reason you're using a text field? I would've thought a dedicated duration field would've been the way to go. I've never used any myself, but Google suggests: HMS Field, Time Period. Have you had a look at them? They might already have a formatter that supports what you want (a longshot, perhaps!).