Did Japanese Buddhists once use sky burial?
I was curious about this question and I found some sources that might answer this question more or less:

...in ancient Japanese society, prior to the introduction of Buddhism and the funerary practice of cremation, the corpse was exposed in a manner very similar to the Tibetan sky burial.

This excerpt is from the wiki article on Excarnation.

The following link opens to a part in an ebook that might help answer this question:

https://books.google.com/books?id=CZM2AQAAQBAJ&amp;pg=PA118&amp;lpg=PA118&amp;dq=Excarnation+~Japanese&amp;source=bl&amp;ots=DF_qKCS7PC&amp;sig=H9zvNsP284SQteLueDyDmyvWmjI&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwjQurPevcDKAhVW2mMKHS6ADKkQ6AEIMTAG#v=onepage&amp;q=Excarnation%20~Japanese&amp;f=false