I have and use ICSP programmers for the tinies I develop but, especially when breadboarding , I include a 6 pin straight header for hooking up a USB-ttl serial cable. This is great for debugging.
Presumably, to be arduino-like, a boot loader could be automatically grafted on to the sketch(like init) and get a shot at the serial before the application started.
Which software do you use on the 84? NewSoftSerial? SoftwareSerial? Something else?
That would be nice. The biggest issue is the bootloader code overwriting itself. There are two potential solutions...1. Always place the bootloader code at a fixed location in the executable image. This may not be possible.2. Make the bootloader so it moves itself to one end of the Flash while the other end is programmed. This would double the wear on parts of the Flash and would be a bit complicated but would very likely work.