The announcement has now been made, and the Switch is indeed featured in the list of "scheduled clients", alongside web, Android, iOS, Xbox and PlayStation.
Rainway works by streaming games from your PC over the internet to any supported device, potentially allowing for graphically stunning games on Switch which would never be possible using the console's processing power. The app is expected to launch on November 25th, with Web and Android being the first two to arrive, followed by iOS and Xbox. Switch will presumably not be available until next year.
How Rainway intends to make its app - which will have to use the console's hidden web browser - available to the public remains to be seen; will Nintendo look kindly on such a product being listed on the Switch eShop? Rainway insists that it is in talks with the platform holder, so we'll no doubt find out in the near future.