No, one character which doesn't fit in the protocol is enough:
Ah. Depending on where it is. The bootloader will accept (nearly) any "command" characters without exiting, but as you say is very particular about getting the EOP in the correct place...
The reason that ArdunioISP wasn't working was that garbage characters (received at the wrong bitrate) were causing the bootloader to keep trying, instead of going on and starting the sketch (ArduinoISP itself.) I think your patch would make that worse, assuming the watchdog is still always reset in getch(). Normally, getting to the sketch promptly when the character stream doesn't look like bootloader commands is a good thing. Any ideas on resolving this incompatibility?