Just got this back from Tier 3 at iCare.
The 4690 IP (H.323) conference phone is not officially supported on the IP Office but many business partners have used it on IPO for years with good results. The problem now is that the 4690 goes End-of-Sale on 9 November, to be repalced by newer Polycom H.323 models that probably won't get tested with IPO either. The reason these products don't get tested with IPO is because they come from the Enterprise Product House.
As IP Office R5.0 now supports SIP-based endppoints, most IOC business partners plan to sell Polycom series 6000 / 7000 SIP conference phones with IPO. The IOC Tier 3 unit has tested these sets with IPO so we know they work just fine with IP Office. But as it is Avaya's stated policy never to endorse 3rd-party non-Avaya endpoints, we still can't bless these Polycom SIP conference phones officially. Personally, I'd go with one of these over the 4690 any day, much better performance.