If you are following Fred's protocol it isn't advised. I had serious dietary issues and malabsorption and tried rice protein and l-glutamine individually and on several occasions at various doses. Each time I found the same thing that Fred has said he and others in his glutathione precursors trial experienced - a return of neurological symptoms and a deficit in methylfolate seen as an inability to absorb mB12 (as seen by discoloration of my urine). Each time the symptoms slowly went away after stopping the protein powder or glutamine.
I noted that at lower doses the symptoms took longer to appear, up to two weeks or so. Thus, it isn't recommended and if you do decide to try it you should watch yourself closely for any effects that might take a while to show up.
The way I solved my need for more protein (I ate plenty of protein but was apparently not digesting and absorbing it as I should) was by getting a plasma amino acid test to show what amino acids I was lacking (all of them) and prescription amino acids (without glutamine or cysteine). These helped me tremendously in my recovery while following this protocol. I continue to take them and follow the protocol and continue to improve. I anticipate that I will not need the amino acids long term...
This was posted on the B-12 - The Hidden Story thread.
Velha was another person with similar glutathione response as I and others had. I trust that this will aid your understanding. Again, like all of us so affected, Velha had good results with the active b12s and folate so that there was something to reverse when the glutathione induced methylfolate, mb12 and adb12 deficiencies hit.