Dear Heiner,
I think none of the three stories of Viśvantara in the Mūlasarvāstivāda Vinaya ("the Viśvantara-Jātaka I--III" in Panglung 1981 "Preliminary remarks on the uddānas in the vinaya of the Mūlasarvāstivādin," p.229) and the Viśvantarāvadāna in the Gilgit manuscripts ("the Viśvantara-Jātaka IV," ibid.) gives aśvaveśa. They do not mention Śakra's help in this part of the story, either.
Viś I (Tibetan, Chinese, and newly identified Skt fragments): Viśvantara gives both the coach and horses to a Brahmin and goes to the forest on foot.
Viś II (Tibetan and Skt fragments): After he gave the coach to some Brahmins, Viśvantara rides his horse. Later, he gives the horse to another group of Brahmins and proceeds on foot.
Viś III (Tibetan, Chinese, and Gilgit manuscript): same as Viś I.
Viś IV (Gilgit manuscript): same as Viś I.
I hope this helps you.