Here is the alpha bootloader
http://85.12.32.240/~superkoe/arduino/attiny84loader.rar
The bootloader works on the first upload with the IDE
Second time upload the chip goes to the bootloader, but fails halfway uploading.
Everything is in ASM so its very hard to debug, this project is on a dead end for me.
I will look to the optiboot, and see if I am more successful.
About serial support, its not very hard to implement. There already some nice soft serials library around, it just has to be changed for the attiny84.
The serial used for the bootloader works aswell, but it doesn't have buffers etc, so you can miss incoming characters.
edit
I just see optiboot already is made support for the attiny84
Lets see if it works 2 