Krusty,
The only explicit regulatory requirement to be PIC on High Capacity RPT is an ATPL. Of course an AOC holder may have additional requirements as a condition for issue of their certificate.
I believe the current CARs allows ICUS to be logged towards the aeronautical experience required for an ATPL in excess of the 150hrs allowed in lieu of actual command time.
I think you are right in your interpretation of Annex 1. However, there is a crucial difference between the Annex (10th Ed) and the current CARs. Annex 1 makes no reference to aeronautical experience or the 50% discounting of co-pilot time towards the experience required for an ATPL. In the eyes of ICAO an hours flight time is an hours flight time no matter where you sit in the cockpit. So once the you have the 500 or 180 hrs ICUS required by Annex 1 the residual up to 1500hrs can simply be co-pilot.
The 10th edition of Annex 1 effective 23rd November 2006 introduced the requirements for MPL, changed the upper age limit for ATPL licence validity to 65 and tweaked the flying experience requirements (eg 70hrs command time instead of 100 for an ATPL). My understanding is that the rule change project underway to ammend CAR Part 5 will capture some or all of those Annex 1 changes. We shall wait and see.
As an aside, under the Annex 1 it will be possible to have an ATPL with only 10hrs command in aeroplanes if you progress via an MPL. Interesting.