[fenwick67]

It would be simple to have hardware that can only use the K-line (which is diagnostic only) or even only uses power from the OBD2 connector.

But you could also design your hardware to be able to write messages on the CAN bus and/or be able to take the bus down.

[cdtwoaway]

Literally: I have successfully sent CAN messages that were understandable to ECUs with an Arduino while waiting for a delivery of real hardware. There are Arduino-GSM shields that are super easy to use and would be remote-accessible.

Such a device would be dead easy to build even for someone who has almost no experience in electronics.

[gh02t]

You can also buy plenty of development boards with an ESP32 and a CAN transceiver. Small, fully programmable, with Wifi and BLE, for less than $50.