"What if instead of sponsoring a field, groups were to set up their own hospitality on this model, make it clear that it's open to anyone who wants to come in (not something that's always perfectly clear) and then people looking for volunteers could just visit each hospitality center rather than sort of canvassing the whole field?"

Because they won't get 'credit' for just doing it.
I did not even receive an AOA for five years of hospitality at my own expense.
Yet the Court and Guard always seemed to know where they could refill Their Majesties' goblets...

It is very important to me that there not be unnecessary layers of protocol and paperwork added to ANY of the SCA. It simply is not needed other than to make an individual or two feel 'important'.

I too like the ability to just walk up and say I'm here to help. We've lost that, to a great extent. I cannot tell you how many events this past year or two I have driven to, paid site fee, gone to see what I could do to help, been told no help is necessary, and then left before noon, in search of some modern-world interesting thing to do...because I wasn't needed, or needed to be in the 'in group' in order to help.

People who are IN the 'in group' never do grok this. There's two fine examples of that right here on this comment page.

t_C, calling it like it is, folks.