Would this be saleable in USA

This is something that would have to be installed by someone else. In my experience as a carpenter the elderly do not like installing their own gadgets. If I were you I would consider a package... such as adding a peephole. I would also look into a more secure chain. Kicking the door in can simply break the chain locks on most homes. Then market your chain as an upgrade, opening the doors to apartment complexes (no pun intended) and more.

As far as a market you could try contacting retirement communities or place adds in local papers. In my area we have "Mature Lifestyles" which is a free publication to those over 50.

If you are ambitious you could try and hook up with security companies and sell to them. They would handle the installation of the product. You could also contact Handyman services and sell to them as well. You could also contact a mail order company like "Harriet Carter" (I think that’s the name) and supply to them.

Just some quick thoughts off the top of my