We have a VSTO Excel COM-AddIn which is deployed as ClickOnce.
This uses post installation actions by implementing IAddInPostDeploymentAction and binding it in the manifest under <vstav3:postActions>.
However we now need a pre-installation action as well (to read registry and disable an existing AddIn).
Would this be possible with ClickOnce?
The only way to do this is to create a prerequisite that does what you want done, and deploy it as a prerequisite. ClickOnce itself does not provide a way to do pre-install commands.
You can use the Bootstrapper Manifest Generator to create a prerequisite package after creating the setup & deployment package to perform whatever commands you need completed.