No, there's no charge for bandwidth. The most common uses so far are users installing datasets locally, which caches the data at the destination or running batch jobs in ECS/EC2, which doesn't accrue charges on AWS.
We welcome your contributions to the R interface. If you email me, aneesh at quiltdata dot io, I can add you to our Slach channel where our engineers can support your efforts. Several users have asked about R and if we combine them together I think we have the horsepower to build an R layer for Quilt.
That would be awesome! We're happy to help, but we only know a little bit of R. We've been looking at Sparklyr in case it might help read Parquet into R.