Some suggestions below about your postcard exchange:
I would write to people individually, through regular USPS mail.
Also, I would post the invitation publicly, in coffeeshops/at school?
The medium of the internet seems to be incongruent with the way you'd like to work with people (hands on, meditative...)
Also - have you considered buying a PO Box for one year, so your home address is not public.
I love my PO Box - it's great for projects like this.
Be a bit careful about the interventions on the front of the postcards - I'm much more interested in the scratched out, whited out areas than the strict representation of what is left untouched. How could those elements work in more communication with one another? I can see you with thousands of these postcards. Why are you whiting out areas and keeping others? What is your motivation behind that work? Why are you writing on top of the writings of others? What is your intention in doing this? Can the two sides be seen at the same time? Do they exist in the same space?
I think you are onto something quite exciting - but I'm a little concerned with the "nostalgic" look of the Dana piece..that could just be my interpretation.
Could the meaning of the postcards change if they originate from other countries, written by other groups of people?
What is the impetus to collaborate?