It looks like WinArchiver can mount 7z and other archive formats. There's not much documentation about it. It does not say if it supports mounting encrypted arc

It looks like WinArchiver can mount 7z and other archive formats. There's not much documentation about it. It does not say if it supports mounting encrypted archives. It does not say if it supports write-access either. The 7z format is normally solid, so unless you're accessing files at the beginning of the archive, it could be pretty slow anyway.