Found a way for crease: 

[![][1]][1]

Defining the **Crease** output in the modifier stack to be a `crease` attribute sets the crease. However this doesn't work by defining **Sharp** as `sharp` or `sharpness` attribute...

[![][2]][2]

Also, the `crease` attribute exists at the beginning, which can be seen in the spreadsheet. `sharp` doesn't...

The specific situation described in my question could be solved entirely by geonoodles:

[![][3]][3]

However it still doesn't completely solve the issue if you want to set edges sharp so that you can use it further in your workflow in a usual way. You (Me? I'm answering myself) could have a simple object consisting of a single sharp edge and use instancing to position those edges and then use *Data Transfer* modifier to copy this information from it... 

  [1]: https://i.sstatic.net/9yMQL.png
  [2]: https://i.sstatic.net/s6DJ2.png
  [3]: https://i.sstatic.net/Og7sp.png