Yes, we've done it for both Tightknit+Slack instances we've spun up now.
You post a welcome video with a short message asking them to react after they've watched, and in the journey, you have people get that step checked off when they react to a specific message with an emoji (you can map it to a specific emoji but we've not opted to do that). It's an honor system obviously, but a pretty elegant workaround.