Custom text expression as palette item

Forgive me if this is the wrong place to ask about this, but every so often I become motivated to search for the answer to this, and so far I haven't found one.

The feature I'm wondering about is how to create a custom text expression, such as the ones that are in the various palettes, with playback options, and have it saved in a palette, or a custom palette, selectable like any other palette item.

Example: Johann Sebastian is writing a composition titled The Broken Wristwatch that features a lot of ritardando phrases. After the end of each such phrase, he selects "Allegretto" from the Tempo palette, places it in the score, edits it to read "a tempo," and then finally changes the text style so that it's in italic type with no boldface. (J.S. is very likely mistaken, but that looks correct to him for some reason.)

J.S. considers that it would be convenient if he were able to create a custom "a tempo" item and save it in a custom palette with his preferred tempo/playback effect and weirdo text styling. Then, after each ritardando, he could simply attach his custom "a tempo" expression and get on with his life.

Is it possible for J.S. to do this? If it isn't, I'd appreciate it if somebody would let me (and him) know so that I (he) can stop looking. Thanks!