dcm2niix is maintained by Chris Rorden. You could open an issue here: [
github.com] though I suspect he will be of the opinion that it's better to spew too many messages than too few. It is packaged with AFNI for everyone's convenience as it is a very popular and useful tool.
Practically speaking, the solution is probably just for you to use `grep` to filter out those warnings. You could try this:
dcm2niix_afni MY_DIR | grep -v "Unknown Manufacturer"
which will only display lines that do not contain the substring "Unknown Manufacturer"
Lastly, dcm2niix is not designed to read compressed files, to my knowledge. The most common solution is to programatically or manually un-archive the dicom files and then run the program.