Ok I found Delta's Point Beyond policy as well
Point-beyond ticketing—use of a fare published for travel to a point beyond your actual intended destination or from a point before your actual intended origin. These specific examples are not exclusive. Using a fare without complying with the fare rules of your ticket violates our Contract of Carriage. In such cases, we reserve the right to:
Cancel the remainder of the itinerary and confiscate any unused flight coupons
Refuse to board the passenger or check baggage
Charge the passenger for the difference between the fare paid and the fare for the passenger's traveled itinerary
Ah well,
So I think that I'll get the BOM-CMB-BOM ticket as well anyway, and make that extra journey while going (leaving the return for another date so I can start a cycle of issuing tickets from CMB).
One other question then - I normally use Thomas Cook (in India) for all my travel. Can they book the ticket out of Sri Lanka, or do I have to find a Sri Lankan travel agent?
Cheers