Dear Patrick,

Despite the broad diffusion of some of Aśvaghoṣa's work,
I do not think that there is evidence of a "smoking gun" proving that
the Buddhacarita was transmitted into Middle Persian and thereby
formed the basis for the very widespread Barlaam legends, which
show up in almost everything from Amharic to Old Icelandic.
And we cannot rule out the possibility that now lost summaries of the
Buddha's life were also in circulation in Iranian Buddhist milieu -- the
evidence of Buddhist textual traditions among pre-Islamic Iranian peoples
is quite fragmentary indeed.

best of luck,