A Lipo battery protection circuit
Apart from using the wrong polarity MOSFET, the basic flaw in your circuit is very soft cutoff because T1 has no voltage gain. However even after fixing those problems it still won't be satisfactory because the Zener will need a lot more than 5uA to regulate properly (and low voltage Zeners also have poor temperature stability). 

Here's the simplest circuit I can think of that meets your requirements. It uses a TLV431 shunt regulator which is 'programmed' by R1 and R2 to cut off at 3.7V (when the voltage on its adjust terminal drops below 1.25V). 



The TLV431 only draws about 0.15uA bias current, so R1 and R2 can be made large enough to draw less than 5uA. When turned on it drops 1.25V between Cathode and Anode, so the FET must be able to turn on with 2.45V Gate drive (the AO6407 is specified down to 1.8V).        

If 'simple' means requiring the fewest parts then you could use a microcontroller such as the PIC10F322. The MCU would be programmed to sleep most of the time (drawing less than 1uA) waking up every few seconds to measure the battery voltage and turn the FET on/off. If your load only draws a few mA then you might be able to power it directly from an MCU I/O pin, and then you don't even need a FET! The MCU could also provide other functions such as auto power off and status LEDs.