If you have access to a TRX system and still want to incorporate a leg exercise like the lunge, no problem. Suspension trainers like the TRX make three dimensional training easier with the ability to modify the demand and difficulty. I'm talking all three planes of movement; sagittal (front/back), frontal (side-to-side), and transverse (rotation).
Demonstrated is the TRX lunge exercise from easiest to hardest...
1) Two hands for support at the bottom of the movement
2) One hand opposite of the leg performing the exercise for support
3) No hands decreasing support from the upper body
4) Added plyometric jump and land
When it comes to suspension training, QUALITY is key because it is so easy to compensate due to the unlimited degrees of freedom. When we compensate, we aren't performing the exercise as intended. When the exercise isn't performed as intended, areas of the body not intended to be stressed might be.