Hi, I looked into this because I was curious, and can offer some suggestions which may not be exactly what you want, or may not work.
Doesn't look like the plugin does this out of the box. The developer, newbthenewbd, hangs out on Discord by that name, and is friendly and helpful, or you could add an issue (I might even do that).
Looking at the TinyMCE customised format example, this example looks like it would fit:
{ title: 'Table row 1', selector: 'tr', classes: 'tablerow1' },
It looks like, in the plugin, you would need to add this into templates/forms/fields/tinymce/tinymce.html.twig. It's better to override templates by copying them to the same path in your theme, so copy this to user/<your-theme>/templates/forms/fields/tinymce/tinymce.html.twig first, then edit that file.
Anywhere in that 'init' area starting around line 26, add something like this:
style_formats: [
{ title: 'My crazy rant', selector: 'blockquote', classes: 'tirade' },
],
(since there don't seem to be any style_formats already set up in that template).
Insert that from line 27 to make it simple.
That should work. Sorry for the blind untested coding. Great feature to add into the plugin though, let's hope! I would totally use this.
Cheers