The current firmware for PiCapture does not implement a “sleep” or “low power” mode. We have considered adding this if it is needed by our customers.
The PiCapture HD1 operates on the video that is provided, and so what is really needed is a way to force your source (Panasonic HC V-770) to 720p mode. If your device honors the EDID information, then one possibility is to use a custom EDID block that does not include 1080p as a valid resolution.
The PiVideo software includes a function to load a custom EDID block. You can create custom EDID files using a software EDID editor such as AW EDID Editor
Here is an EDID block that may work for you:
Not all source devices will honor the EDID settings, and if your device does not support 720p it will default to some other resolution.