Is there an intent for telling Play Store to update all apps, or turn auto update on? I own a Samsung Galaxy S2, running the official ICS OS. I want to use the Llama app (too poor for Tasker!) to tell my phone to update all apps whenever I am connected to certain Wi-Fi networks (but not all - my home Wi-Fi is volume-based).

Llama allows you to signal custom intents, but I don't know how intents work. Does the Play Store support an intent to either signal "update all", or to turn on automatic updating?

(Essentially, I'm trying to do something like this: How to allow auto-update of apps in Play Store only when plugged in?)
"to turn on automatic updating?"

I highly doubt there is an intent for this - that is a user setting and Android doesn't generally allow apps to change user settings. 

I think the best solution is to, first, manually turn on the auto-updates feature of the play store, and set it to wifi only, and second, launch the play store when you connect to the desired hotspot.  Just launching it will typically cause it to perform any pending updates.

I'm not sure how you specify this in tasker or llama, but you can launch the play store by sending an intent that requests that a play store URL be launched (e.g. the URL of a play store app or publisher), e.g.

new Intent( Intent.ACTION_VIEW, Uri.parse("http://play.google.com/store/apps/details?id="+appName) )

More info about that is here.

Finally, you should go to the wifi settings and mark your home wifi network as metered.  Whoops.  I've seen that setting in the API docs here but now I can't find a way for a user to set it.  Maybe that is only available for cell networks, or maybe that is coming in future versions of Android (the API was just added recently).