So, if a TM aquires a Datajack, a nice implanted one, and plugs that into a device he wants to connect to, say, a host, can he still thread?
The only reason for a TM to
ever get a datajack is if she for some reason want to use a device based persona rather than her living persona.
A host is a matrix-only entity. You cannot plug your datajack (or rather your cyberdeck) into a host.
A wireless enabled device can be slaved to a host. In that case the device use host ratings when defending (unless you are directly connected to the device). If you get a mark on the wireless enabled slaved device then you also get a mark on the master.
A TM that want to establish a direct connection with a wireless enabled slaved device to bypass master ratings would either enter the host (you are considered directly connected to all devices slaved to the host) or get the skin link echo and physically touch the device.
A third option would be to go old school.... Get a cyberdeck, base the persona on that instead of using the living persona and connect the cyberdeck (not the datajack) directly to the device (with a wire). This can be done from AR perfectly fine without a datajack (using image link, AR glove and ear buds). With a datajack (or trodes) it can still be done in AR but it can also be done from cold- and hot-sim VR (without image link, AR gloves and ear buds).