While only folks from Adobe are capable of answering
To the best of my knowledge, indeed, there’s currently no practical way to add videos or iframes to plugins.
May I, in part out of curiosity and in part since it might lead to devs having faced similar challenges to share their solutions, ask what your Use-Case for having videos within your plugin is?
While I haven’t used Lottie a lot, before, and am not too familiar with the technologies behind it, fillStyle looks like a canvas API (CanvasRenderingContext2D: fillStyle property - Web APIs | MDN). Canvas elements, like video and iframe, aren’t supported by UXP, as of right now (a somewhat related topic that also mentions canvases is Draw image using UXP).
I’m not sure if UXP allows for alternative rendering methods where, perhaps, some might work in UXP… But since any “more complex” animation library (such as Lottie appears to be) has a heavy focus on running in browsers using some “more advanced” browser features for performance optimization, I, unfortunately, don’t believe the chances of getting it to work are high, as of right now.
Again: If you could, perhaps, write about your Use-Case, somebody might be able to share a solution 
I hope that this helps, even though it’s probably not the answer you were hoping for 
Happy Coding,
Pablo