"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).