No, there is no standard specification for performing a firmware update to ANT+ devices at the moment.. Some products use ANT-FS for the file transfer, but specific parameters (e.g. when is updating the firmware enabled, what index to upload to, the format of the file) are defined by the manufacturer.
Regarding manufacturer specific requests - well, they are manufacturer specific... :) ANT+ defines a set of data page numbers that can be used for manufacturer specific purposes, but the actual content of the pages and its usage is defined by each manufacturer.
