Hi @theoriz, is that license key some special device, or just a storage device?
If it’s the latter, you may be able to multipass mount /media/$USER/<label> <instance>:, it will get mounted in the same location.
If it’s a special device, you may be able to do what you need with libvirt, I just added a section in our documentation about it.
You’ll have to run something like snapcraft pull first to have Snapcraft create the instance and modify it afterwards. Note your changes will not survive snapcraft clean as that’s removing the instance completely.
Please also file a bug with Snapcraft so they’re aware of this requirement and can maybe design a solution.