Following, the script could work as follows:

1. Extend refraction line to transition surface-or-line
2. If extended successfully, proceed
3. Get normal from transition surface-or-line
4. Create new line to refract
5. Scale sine of that line to the normal by refraction transition index (typed).
6. Mirror that line along transition surface and normal

I don't know how to write scripts for MoI, so someone maybe wants to do it.