I have made a Multi-catagory tag with shared parameters that does something similar to what you want (I think). The tag has three types - Existing, Demo, New. The shared parameters are added to the project and assigned to all model categories.
There are 6 shared parameters -
1- Existing Text (type)
2- Existing Text Additional (Instance)
3- Demo Text (type)
4- Demo Text Additional (Instance)
5- New Text (type)
6- New Text Additional (Instance)
Now in the project I can tag any item with a multi-catagory tag and assign the text at a type level and additional text to any instance of that type. I just add different text to each phase as required. It doesn't have the controllability of a text box but it works ok in most situations and if I need to change any of the text it updates globally.