I'd like to integrate Marketo and Slack by posting the number of leads in a static list in a given Slack channel daily. To do this, I think I need to use webhooks and/or Zapier. But...without an API call to get the number of leads in a list this isn't possible...right? And I can't see that particular API call in the Developers API pages.
Is there an easier way?
Doesn't exist yet.
Depending on your definition of "easier," sending Smart List subscription to an email processing engine is another option.
Could you clarify what you mean by "email processing engine"?
Something like Email2DB: an SMTP server that can parse emails with a known structure and pass info into other systems, for example reading data from a subscribed Smart List and posting it elsewhere.