No, there's no way to CW a boost because it might be misused to quote toot:
https://fedi.tips/how-to-use-content-warnings-cws-on-mastodon-and-the-fediverse/
One suggested workaround is to have a CW reply to the thing you want to boost, and boost that reply.
Regarding videos specifically, they really ought to have their warning at the start of the video itself rather than just in the description. Videos that are embedded will not have a description, and warnings there will be missed.