I've checked out the project from Gitlab and implemented the feature locally for me.
I was waiting for some feedback from an active developer to see how to oficially contribute with this feature, but I can imagine there is more to it than simply updating the mediaitem.py in the songs plugin and starting a marge request.
I assume I would need to add a setting and than some type of configuration and make it gereral enough that it could cover many use cases, also add the formatting to the default formatting, and so on...
If @raoul or any other mod could would get in touch, I'd love to know how to make the contribution for the version 4.0.
Otherwise I can check to see how to share this with you newidea . But no promises.
-- Edit --
As an alternative, you can manually add the line you want to preview between any kind of annotation, like {preview} Line to preview {/preview} and add add this custom format to your formatting tags like this:
<span style="opacity: 0.6; font-size: 80%;">
But it is a manual work that will be saved to the song, which I did not want.