I am happy to report that the CA-3 is fully compatible. On my IXUS 960 IS, I set the Switch Type to 'CA-1' in Remote Parameters and it works like a charm, both half-press (you actually just touch the CA-3's button very slightly) and full press for shutter release.
In the end, I had to buy the CA-3 to test it but my friend, the Ricoh GRIIIx owner, said he might buy it from me in case it didn't work with the Ixus. Well, he will have to get his own CA-3 cable release now...