Splice high current charge source into USB cable?
While it is possible to do something like this, there are 2 immediate problems I see:

1) Your device no longer has a common ground with the computer. This will likely result in your computer not 'seeing' the device, or complaining that it cannot communicate with the device.

2) Assuming the above was not an issue, certain phones (Apple products for sure), have a unique method to detect if the charger they are plugged into supports high(er) current charging. As you mentioned, the USB standard officially supports up to 500mA charging. The way Apple handles this is by adding a resistor divider on the D+ and D- lines (in their charger accessories) to bias the DC voltages to different values. The phone is able to detect the voltage ratios and then determine the type of charger it is plugged into, activating the supported charging mode.

Here is an image of the resistor divider that was used in a nifty product:



To find out more about, check out the project on ADAFruit: https://learn.adafruit.com/minty-boost

So in summary, it's not as simple as your schematic. As others have suggested, your best bet would be to look into USB hubs that support high-current charging.