I use often “dinamica ad libitum”. I would be glad if the term could be added to the dynamics.
What about a text playing technique ?
That’s what musicians do, even when instructed otherwise. 
If “dinamica ad lib.” changes with usual dynamic symbols it would be useful to have it (with a font which likes on the fonts of usual dynamics) also by the other dynamic symbols.
Dear Maximilian,
You can write it as a prefix or a suffix of any “default” dynamic, in the properties panel. Make sure you Hide dynamics marking just above those fields, and use properly the positioning on the right of those fields. Of course, the playback won’t follow this unusual dynamic, but it will show perfectly in your score.
Thank you, Marc!