accidentally I've just discovered this -- Falcon RS232 output is not caught at all. It is easily reproducible with FreeMiNT snapshots, for instance download https://tho-otto.de/snapshots/freemint/bootable/freemint-1-19-ab9c65b5-02060-tt_falcon_clones.zip, disable machine/falcon/scc..xdd, move mint030h.prg into AUTO (and remove the existing PRG there), reboot, press shift, set the debug output device to AUX: and debug level to DEBUG and... see how nothing happens.
If I change it to MID (and change Hatari settings accordingly to MIDI output), I see debug messages. If I change the machine to TT, I see debug outputs even from RS232. So FreeMiNT works OK, Hatari works OK but just not on Falcon. :)