You can add a reference link to any site you like, including the ones you’ve listed. However, I think you’re referring to the fact that lyrics from the ToneWay Project and Bluegrass Lyrics can be opened in an embedded window – which, by the way, will also be viewable in a column next to the chord chart in the future.
The reason I am able to do this is that I have permission: Bluegrass Lyrics is run by my old friend John Kael who was nice enough to let me pull lyrics from the site, and the ToneWay Project is my family’s website.
The Lyrics.com Terms of Service forbid this kind of thing quite clearly, and as a giant website I’m sure I have no chance of getting permission – there’s no upside for them. However, the other two sites you link to are one-man operations (like Strum Machine) so they might be open to the possibility – I suspect that they, like me, are ultimately in it for the music and the community. I will contact them both and see what they say.
I’m definitely open to suggestions for other sites I can embed as references. Beyond the lyrics sites you mentioned, you should be able to embed Google Docs pages, as well as video links from YouTube and Vimeo, and audio links from Spotify, SoundCloud, Apple Music, Clyp, and Dropbox.