I tried the test scripts and when it opend a video, it opened my media player and it went on in the background.
as an alternative, can there be an iframe in the script that opens an embedded youtube or xhamster video?

There are no iframe.
Embeding movies was tried 2 years ago ; I'll give a try to Vlcj in a few days, to see what can be done.

I tried some things with vlcj, but it's too invasive, and too risky / hard to maintain into SexScripts (native dependencies, etc.).
Anyway, this would not have done what you mean, it would have been necessary to download the video, then play it.

The best solution now is to write useUrl("http://...xhamster...");. it is simple and will works for everybody.

Nearer to iframes, there are some swing browsers available ; I'll give them a try to see if they are light enough to be embedded and advanced enough to show the very complex pages that are on video websites. I won't be too optimistic.