Currently there isn't a way to silently preform the blank installation, since the process requires someone actually cancelling out of the registration prompt. This is a workaround rather than a feature.
The registration command has changed in 9.2. Assuming you're using the shared passphrase authentication, the command you would want is:
"C:\Program Files (x86)\LANDESK\Shavlik Protect Agent\STAgentManagement.exe" -register -p silent=true -p authType=SharedPassphrase -p passphrase=<yourpassphrase> -p serverURI="https://ourserver:3121" -p policy="Scan Only"
Other authentication types are available, and will need to be specified if used.
The property argument is property=value, which is why your silent argument was invalid. It was never assigned to true or false.
A full list of commands and syntax is available by running STAgentManagement.exe without any flags
Worked great! Thanks for the information.