I know that I can use the CLI command si viewrevision to get the content of a versioned file. Downside is that this file must not be dropped.
Does anyone know a way (other than addfromarchive) to get the content when knowing the archive?
I don't believe this is possible
si projectco is documented as "checks out members of a project into working files". If you drop the member from the project, it is no longer part of the project.
At first blush, si viewrevision doesn't explicitly state in the documentation that it requires a project, but if you try to run the command without a project (or a sandbox, which implies a project), you will be prompted for one. Failing to provide one at the prompt exits the command with the message 'A value for "--project" is required.' I tried doing this specifying a change package ID which the member was part of, and that still doesn't work.
Your si addfromarchive option is the only published way to do this.
Disclosure: I am a PTC employee.