I must determine if you can access the client's hardware via internet browser basically produce a wordpress plugin or extension. The customer accesses an internet site (via cloud) and also the wordpress plugin is responsible to acessing his serial port, parallel port, etc, and delivering client's information towards the cloud for processing.

I know, the opposite can be done: Write a Winforms application, that only consists of a browser and also have it talk to the serial port without anyone's knowledge - this is the way I access a ticket printer inside a ticket sales application.