Added configurable parity control. Parity can be NONE, EVEN or ODD, selectable by contents of address 0F96DH which RS232 Queue Putback flag that is not used by MSX BIOS.
The hook code size is 63 bytes and can still fit within unused RS232 Queue space (64 bytes). The entire binary takes 93 bytes.

Parity was checked using oscilloscope and a BASIC program .

10 REM test transmission with parity
20 CNF = &HF96D
30 REM
40 REM 0/1: no parity   (N)
50 REM   2: even parity (E)
60 REM   3: odd parity  (O)
70 REM
80 POKE(CNF),2
90 LPRINT "A";
100 GOTO 90