Hi Adrian! Unfortunately, no customization is possible (yet) during online check-in.

You may consider sending the After check-in e-mail after actual check-in (on-site check-in) to send your guest a welcome message. 

Regards, Sanne 

Hi Adrian! Only at the kiosk is a screensaver video supported at the moment. For online check-in on other devices there is no video integration provided so far.
Best regards, Richard

Hi Adrian

As Sanne and Richard said, it doesn’t look like you can configure a video to play as part of online check-in. You can view the online check-in journey here, so you can see what it involves. It might be worth suggesting this as a feature to add via the Mews Product Forum - I’d give it an upvote!

Sami