I think that will definitely be my plan if the pH is an issue. I'm not in too much of a hurry and am willing to experiment a bit because my small salamanders still seem perfectly happy in a 10 gallon (though the Ocoee is getting big amazingly quickly), so I will probably at least give the cement a try.
Right now I am in the midst of coating most of the stream in black silicone anyways because of a handful of small leaks that I cannot seem to find on the top side of the stream. I thought my foam layer under the concrete was fairly seamless after all the silicone I used, but it turns out that I must have missed a few cracks. The drips that emerge are fairly slow, but I think coating the stream with silicone now and then covering it in another layer of cement should make it mostly waterproof.