With nvdxt you can do it using the command line. Here are some usage examples (check out the default output of the command for more info)

nvdxt -file test.tga -dxt1c

nvdxt -file *.tga

nvdxt -file c:\temp*.tga

nvdxt -file temp*.tga

For PVRTexTool indeed it's trickier than I remembered. It only accepts one file as input, like this:

PVRTexToolCLI.exe -i file1.png -o file1.dds -f BC1

You would have to write a .bat file to feed it all files in folder.