There is a tid bit when you define a Custom Installation Path:
After the package is extracted, it will be copied to this location on the remote machine.
Where is the initial extraction path?
I’d like to do some file comparisons and backups on files before deploying them, and basically want to get at the contents of the package first.
Hi Matt,
Thanks for getting in touch! I have some information and links which may be useful for you here. When Octopus deploys your package, it will create a new directory each time, the default being:
C:\Octopus\Applications\[Tenant name]\[Environment name]\[Package name]\[Package version]\
Where C:\Octopus\Applicationsis the Tentacle application directory you configured when installing Tentacle
After the deployment has finished to this folder, the copy will take place. I can give you a link to our documentation which has information on the exact ordering of the package steps here
And for some further reading on deploying packages.
Let me know if you have any further questions after the above. 
Best regards,
Daniel