When you have a serial connection to a filer, you have essentially a terminal session to the console. You can use telnet from PowerShell to send commands and then strip and parse the reply. This would work for one off situations where there is no Data ONTAP PowerShell cmdlet, like mucking around with the boot loader. You could of course run any console command from the session.
The Data ONTAP PowerShell Toolkit wraps ZAPI, which communicates via http/s. The API is not available from the console. It would be a monumental effort to rewite the entire Zepher API to send commands via the terminal then strip and parse the output. It would be, in effect, creating an entirely new API with the same functionality as ZAPI.