Like You may be would know had the ancient Tibet from before 800 a script which is called the Zhang Zhung Script.
After the great reformation etc. there was suddenly need of another script which was based on the Sanskrit language spoken or written only in Swat / Uddhiyana where the great Mahasiddhas, Naths lived.
Based on Sanskrit there was developed the new Tibetan or the Sarma language which counts untill anno data
All in all is the Zhang Zhung language spoken and written in the 18 Zhang Zhung kingdoms. These kingdoms were situated nearby the Silk Road (western Tibet) and so this language from Bon was wellknown.
Personal don't know if the rest of Tibet had a script like Central Tibet and eastern Tibet, but i guess of no.