I have build one! IT is working, but got two issues.
1. The detector diode are working in non linear part of the conduction curve.
2. The DDS has 200 ohm impedanse so the output is to low.
Because of this the swr is not 100% correct.
Currently Im working on a remedy for this.
1. Change the bridge to a propper return loss bridge with an AD8307 logamp. Some chance must be done in the software to go from return loss to SWR.
2. Add a buffer amp with some gain and 50 ohm output.
In additional to this Im putting in a AD8307 power detector to sweep filters and other circuits.
The unit will be more like a VNA.