The New LiquidCrystal library is not limited to any aparticular HW wiring or driver as far as pin out is concerned. The base constructor does assume a particular wiring for all the drivers it is capable of handling, but every class has constructors that enable users to configure the pin mapping of their LCD.
You just need to look at the header files for each driver and see the flexibility.
As far as the backlight is concerned, once again, the default constructor does not assume support for the backlight. However, the I2C flavour of the library does support the control of the backlight, you just need to invoke the correct constructor when crating the LCD object. Again, you need to look in the header files to see which method suits your needs.
Will it be extened to support other I2C IO extension ASICs, it will. Currently it supports the PCF8574 and compatibles which is very much standard.
The other thing that is very nice about the library is that you just need to add a new class, with a few methods to support any other driver you can think off (just need to pass how to write to the device and control the backlight). The rest is done for you.