Potentially answering my question, there appears to be a few options.

1. The platform_channel documentation points out method calls can also be sent in the reverse direction, with the platform acting as client to methods implemented in Dart. A concrete example of this is the quick_actions plugin.
2. It looks like a BasicMessageChannel’s send() method can be used to send the specified message to the platform plugins on this channel.

Creator of Coinverse - The 1st Crypto News Audiocast App @ bit.ly/play-coin

Creator of Coinverse - The 1st Crypto News Audiocast App @ bit.ly/play-coin