How to automate releasing for Scala.js 0.6 and 1.0

[exoego]

In Scala.js, for cross-publishng to 0.6.x and 1.0.0-RC, it is common practice to use system env virable to switch Scala.js plugin version.
One example can be found on circe.

For Scala.js cross-publishing, one will performe the steps like below:

1. Set system env to 0.6.31
2. Start sbt
3. Release to 0.6.31
4. Shutdown sbt
5. Start sbt
6. Set system env to 1.0.0-RC1
7. Release to 1.0.0-RC1
8. Shutdown sbt

Any idea to automate such process in single releaseProcess settings?
My understanding is that it is impossible because sbt can use/switch multiple version of scala-js sbt plugin in single instance.