OK, looks like I'll be soon ready for "self-sacrifice".
I'm finding out that yes, there is ROMchecksum function, likely being called on 2 event procedures under "RomCheckSum" and "CheckSumAll" names. In fact, RomCheckSum (at ROM:FF818190 on TX1) is getting called from CheckSumAll (ROM:FF81822C SomeChecks)...
Event creation is here:
ROM:FF817F50 LDR R0, =aGetfirmwarever
ROM:FF817F54 LDR R1, =sub_FF817FE8
ROM:FF817F58 BL ExportToEventProcedure
ROM:FF817F5C LDR R0, =aRomchecksum
ROM:FF817F60 LDR R1, =RomCheckSum
ROM:FF817F64 BL ExportToEventProcedure
ROM:FF817F68 LDR R0, =aChecksumall
ROM:FF817F6C LDR R1, =SomeChecks
ROM:FF817F70 LDR LR, [SP],#arg_4
ROM:FF817F74 B ExportToEventProcedure
...
As I need to disable those 2 functions before "injection" and changing ROM, now I have a choice of either overloading aChecksumall and aRomchecksum event pointers (as DataGhost suggested above), or as I'm planning to "inject" anyway, I migh just first "inject" proper code into RomCheckSum, and shortcircuit it so it will always return that checksum is OK or just remove calls to those Event creation completely by writing just 1 byte to ROM at ROM:FF817F58+3 replacing value of that byte from 0xEB to 0xEA, thus forcing not to execute Checksums event procedure registration. Effectively asm will look like:
ROM:FF817F50 LDR R0, =aGetfirmwarever
ROM:FF817F54 LDR R1, =sub_FF817FE8
ROM:FF817F58 B ExportToEventProcedure
ROM:FF817F5C LDR R0, =aRomchecksum <= not executable portion now....
ROM:FF817F60 LDR R1, =RomCheckSum
ROM:FF817F64 BL ExportToEventProcedure
ROM:FF817F68 LDR R0, =aChecksumall
ROM:FF817F6C LDR R1, =SomeChecks
ROM:FF817F70 LDR LR, [SP],#arg_4
ROM:FF817F74 B ExportToEventProcedure
...
I might be brave, but looks like I need a spare FLASH though.LOL