I think you have done a good job of identifying some of the key roles when it comes to influencing change within an organization. One of the things to keep in mind is that not only do you need to be aware of who these people are but also of the nature of the relationship that you have with them.
True social capital goes deeper than simply knowing somebody (or being connected to them on LinkedIn or their friend on Facebook).
I know that I have personally been in situations where I have focused a great deal of my energy on having a quality relationship with the Don and many of the Influencers, but have taken my relationship with the Inner Circle folks and Loose Cannons for granted.
Those people sink ships. Frequently.
If you truly want to drive change you have got to be relentless in building and maintaining social capital and those efforts need to include all four groups to some extent. Social media / social networking tools are great for this, but within the organizational setting, nothing beats face time.
Are you in the habit of requesting face time on a regular basis with the Don(s), the Inner Circle, the Influencers and the Loose Cannons? Are you inviting them in to observe your department meetings from time to time? Are you sheduling departmental open house events, where folks from other parts of the organization can network with your team and get updated on what you are working on?
I think that HR leaders that are really going to be able to drive change are those that are constantly finding ways to weave themselves and their team together with other parts of the organization. It has to be a constant effort, so that the social capital you need is there when you need it.
Social Capital: HRs Secret Weapon, Part 1
Social Capital: HRs Secret Weapon, Part 2
Social Capital: HRs Secret Weapon, Part 3
Social Capital and Corporate Politics