Solution:
As of WP-CLI version 2.5.0, the separator to be used between the package name and the desired version is a colon:
wp package install wp-cli-configmaps/wp-cli-configmaps:1.0.1
To verify it worked correctly, use the wp package list command – you should see your specified version in the output:
+-------------------------------------+---------------------+---------+--------+----------------+
| name | authors | version | update | update_version |
+-------------------------------------+---------------------+---------+--------+----------------+
| wp-cli-configmaps/wp-cli-configmaps | Bostjan Skufca Jese | 1.0.1 | none | |
+-------------------------------------+---------------------+---------+--------+----------------+
The relevant WP-CLI source code is here. This probably works with pre-2.5.0 WP-CLI versions too.