Even the ISG thinks it won’t work
There’s this passage on page 52 of the report:
Our limited contacts with Iran’s government lead us to believe that its leaders are likely to say they will not participate in diplomatic efforts to support stability in Iraq. They attribute this reluctance to their belief that the United States seeks regime change in Iran.
Nevertheless, as one of Iraq’s neighbors Iran should be asked to assume its responsibility to participate in the Support Group. An Iranian refusal to do so would demonstrate to Iraq and the rest of the world Iran’s rejectionist attitude and approach, which could lead to its isolation. Further, Iran’s refusal to cooperate on this matter would diminish its prospects of engaging with the United States in the broader dialogue it seeks.
So even the ISG acknowledges that it may be that nothing much comes of outreach to Iran.