RS232 Splitter Capable of Transmitting/Receiving data to one device and only Transmitting to another
So, something like this?

TX1 ------- RX2
RX1 ---+--- TX2
       |
       +--- RX3


where TX1/RX1 is the ECU, TX2/RX2 is the radio transceiver, and RX3 is the DAQ?  That should work just fine, presuming all of the voltage levels are identical.  You'll probably need to make a cable or adapter for this, though, but it's completely passive - just connectors and wires.