If NES would be in sync. with PC, in theory we could use zapper ports to output
an 8-bit serial data just like controller. Since the situation is not in our
favor, we can only output a single bit. These bits could serve purpose in
PC to NES communication attempt, what do you think?

Zapper sprite detection may have a huge tolerance, for this reason it may
be that it's not very ideal for serial data transmitting.

Zapper trigger/sprite is quite simple to utilize, only one transistor for
each is required to do this. BC547 transistors are quite cheap they should
be around 10 cents, not a reason for bankrupt i think.

PCB would be the best way to build this, but unfortunately not everyone has
equipment for this.

Building this device on experiment circuitboard will is probably the best
choice, they are not too expensive either, around 2 bucks.

I don't have very much experiece from NES asm yet, so i cannot provide
my help in that i'm afraid... do you have ideas for software on NES?
I guess somekind of operating system or terminal program is pretty much
required.

What are your future plans for this project? NSF maybe?