Is there a tool that can perform a read-group-aware mpileup from a single file?
I broke down and finally scratched this itch. I have implemented a tool that performs read-group-aware text-pileup from a single SAM/BAM file. The tool is called streaming_pileupy and it's available from pypi for installation with pip. After installation the command would be:

spileup input.bam sampel_names.txt


The tool is pre-alpha and missing some features. Contributions are welcome.