Level of involvement for an org.’s agile leader when there is need to hire new dev team members

What responsibilities do you think a Product Owner ought to have for the above?

Thanks Ian (knew you would likely post a question :)

I would imagine a PO would be more involved if they had straight line budget control of Dev Team headcount and related expenditure, or if there was need to hire a Scrum Master that they would then liaise with closely.

But in this case the focus of responsibilities I'm keen to consolidate knowledge on is the Agile Leader, as we know the PO has a different responsibility scope (not servant leader type).

If I interpret @Ian's question correctly you may missed the point. In the thread you referenced many people included the PO role as being responsible for communicating the product's direction. Then the rest of the organization can decide how to form the team to satisfy those needs. 

To your original question, I think you have the basic idea of how an agile leader plays a role. But ... 

• financial sign off for the role(s)

Not sure that "sign off" is the best choice of words.  A real agile leader will support the decision and not sign off on it. And a truly empowered agile team would not make the decision to hire without merit.  Agile organizations will trust that the team is making this decision for the right reasons and not just to get more people on their team. The organization will also trust that the current team has assessed and tried other options in order to arrive at the decision that more people are needed. 

• give the nod for HR assistance in the process

Again, an agile leader really shouldn't be involved at this level. They would have already delegated the authority to the team to initiate contact with any section of the organization that is needed to accomplish their goals.

Your other two points are not too far from my viewpoint. 

There is no clear definitions of self-organizing teams.  There are a large volume of interpretations and suggested practices.  But the only real consistent element is that the team is empowered to make the decisions that they feel are appropriate and that the organization as a whole respects them to do so. That thread you referenced did not provide a clear viewpoint because I don't believe there is one.  It did provide a lot of perspectives from which to view the issue. That is agile in its best form. 

One technique that I have used to clarify who is responsible for what is Delegation Poker from Management 3.0.  This can be facilitated with Management and a Scrum Team, or within the roles of a Scrum Team.  Delegation Poker has the potential to foster great communication.  You can print the cards and play it like Planning Poker.

In this case delegation could be one of seven levels:

1. The Manager makes the decision on who to hire and tells the team about the decision.
2. The Manager makes the decision on who to hire and sells it to the team.
3. The Manager makes the decision on who to hire but first consults the team.
4. Both the Manager and Team agree together on who to hire.
5. The Manager advises the Team but lets them decide on hiring.
6. The Manager lets the team make the hiring decision and only inquires when a decision has been made.
7. The tManager has fully delegated the hiring process to the team.