Yes, using OpenPlotter GPIO you can take a feed from your SeaTalk network and bring the data into OpenPlotter. You can't go both ways, just Seatalk -> Openplotter (SignalK). You don't need any sort of multiplexer, just connect to the SeaTalk bus.
It requires a fairly simple signal level converter to bring the 12V Seatalk to the TTL levels that GPIO expects. These work well, but you do have to change a resistor: https://smile.amazon.com/gp/product/B01L1OI1HC/?th=1
It requires a fairly simple signal level converter to bring the 12V Seatalk to the TTL levels that GPIO expects. These work well, but you do have to change a resistor: https://smile.amazon.com/gp/product/B01L1OI1HC/?th=1