So.. for getting track data from the empeg:
Two scenarios:

(1) Arduino manages BT module: Hijack needs to intercept serial input, and respond to requests for track info, otherwise leaving the serial port as-was (passing everything else to the player).

(2) Empeg (Hijack) manages BT module: Hijack can safely dedicate the serial port to only the BT module; no need to worry about player commands or anything other than BT passing through.

EDIT: Or perhaps somewhat simpler for number (1): Hijack could just periodically broadcast track info over the serial port, unsolicited. The Arduino could listen for this and cache it for later response if/when queried by the head unit.

Does the Arduino sketch currently wait-for/read anything else on the serial port?