This is not about VNA and reflections and so on. The 100mA is DC as stated, so all you need to ensure is that the load is 50 ohms accurate to 0.1%. With DC, this could be any low TC 50 ohms resistor (or e.g. two 100 ohms in parallel, for practical purposes). You can verify the accuarcy with the 3458A directly. Then you can adjust your amp accordingly.
You could then, in a next step, transfer the now known DC value to the load you have (and derive a factor) and move on with AC (probably your DC resistor, if reasonably good would also work considerably high at AC).