Why don't you want to pick a solid face to set the UCS.
Do you want to preset the UCS before you run the program
![Huh ???](https://www.theswamp.org/Smileys/black/huh.gif)
Do you know the best way to select the Face ??
The 3DSolid face data is not available to Lisp or VLisp so attempting to use entsel or entget is useless.