Further musings...
It took me a while to figure out how disconnected Marketo and ReadyTalk really are. The statuses of Invited, Registered, No Show and Viewed Recording are very much dependent on status changes being handled entirely in Marketo and do not reflect the very similarly named statuses in ReadyTalk.
A close reading of the Marketo/ReadyTalk guide bears this out in that the only data communicated back to Marketo is attendance.
So, in terms of "Watched Recording", it appears there are a couple of ways to handle it:
1. Place your recorded webinar on a landing page and require form fillout to access it
2. In email, link to the ReadyTalk page for the recorded webinar and track clicks on that link
3. Export the viewed recording list and import that list into Marketo for the associated event campaign
Option 3 has potential for data loss because users who go to the view recording page at ReadyTalk are able to enter their name and email address anew, which could result in somebody giving false (or just different) info compared to what they used for registration.
Option 2 works only if your invited/registered lists reside in Marketo (otherwise you'll just have the Attended list, and, well, those people already saw your event).
Option 1 is similar to option 2 in that way - you need to have enough data in Marketo to know to whom the recording page should be sent, unless you're using the recording as a lead generator on your site and don't mind getting a bunch of new leads. However, it wouldn't be fair really to classify those leads under your specific webinar event since they might come across that page any number of ways.