You have to have access to the SDA pin to do recovery.
You will need to modify your design. Floating SDA is not the way to go.
It has been recommended that SDA be connected to ground instead of vcc when powering on for Recovery - safer for the chip.
I have found the recovery to be more sensitive to variations in the USB cable. Some of the cables we built cannot perform recovery successfully.
Keep the cable short, I have not been able to recover with a cable longer than about 9 inches. Keep any solder connections "nice". Do NOT use a breadboard for this. I was not able to recover until I ditched the breadboard and made a short straight cable from the USB to the module.
Steps:
1. power off board (e.g. disconnect USB cable from PC)
2. connect SDA to ground
3. Power up board while SDA is grounded
4. Wait a couple or three second
5. Disconnect SDA from ground
6. Double click the Recover target in the SDA.
This works for me. We use a FTDI USB to RS-232 3.3 volt cable. It has LED's in the USB connector and you can see them flicker when the download/recovery is in progress.
