We know thanks to a Greek document like
The Periplus of the Erythraean Sea that there was extensive commerical activity along the Somali coast and multiple port towns. And writing is needed for record keeping for trade ,business, and to keep lists, inventory etc . And we soon get a confirmation on this
by a Roman writer about one of the port towns:
Ten days rowing , further south , is the port of Isis , near which are stone pillars , with unknown letters upon them
We also know through archeological observations there are inscriptions across the interior of the region. In Laas Gaal, near Hargeysa, there are as many as 600 inscriptions in the scene; and in the Lag Oda, near Dirir Dhabe, there are about 800 of them. But these are only a few places that are investigated.
They are also all over the different place inside Somaliland & Puntland found on different Taalos(Stone mounds) , rocks and ancient burials etc , and in other caves.
Wouldn't be surprising if they wrote on some earlier Musnad script, as some inscriptions have been found but i'm uncertain if these are the same ones. It makes sense considering the geography and their neighbors. But we don't know the language content of it
Some more research and investigations need to be done on this, to know more.