WIFI SD card for a Linux embedded device?

I'm researching the possibility of adding the capability of wireless file transferring, to an embedded device running Linux.

It seems to be that a shortcut solution is to replace the SD card on the device with a wifi-enabled SD card, one like eye-fi.

The application will be a peer-to-peer type: one device to one client.

My questions are:

How the device controls the on/off of the wifi? (via Linux driver?)

How is the file owndership handled? (are file shared by the device and wifi, or the device need to unmount the card before enabling wifi?)

What's the status of the client side API? (is it easy to fetch the files from the wifi card in an existing client side application?)