I simply took ts-bios.rom and looked at it using "hexdump -C" under Linux. It revealed that the 4th 16K is the 48K ROM, it was easy to overwrite that part with "dd". I flashed it into ROM and gosh, it's working almost wonderfully.
I have to select ROM #04 / Basic 48. Selecting ROM #04 / TR-DOS doesn't fully work because the default entry mode of this modified ROM is "L" mode and while manually typing LIST and GOTO "filename"CODE works in TR-DOS, RETURN does not. I have to switch to "K" mode with the STOP statement before entering TR-DOS. It works then fully.