I haven't thought about whether this is sufficient but if it is then an initial useful thing to add would be to use it for queries where we have a qual that can be checked using the index key even though we can't do a range scan. For example if you have a btree index on <a,b,c> and you have a WHERE clause like "WHERE c=0"
That would be a much smaller change than IOT but it would still be a pretty big project. Usually the hardest part is actually putting the logic in the planner to determine whether it's advantageous. I would suggest waiting until after 9.0 is out the door to make sure you have the attention of Heikki or Tom or someone else who can spend the time to check that it will actually work before putting lots of work coding it.
