How about bmaptool? It can work on sparse images.

In general, I don't see the point in manually specifying block sizes most of the time. What I want nearly always the maximum performance, and that should be possible to derive from the hardware, and/or benchmarking.

What about uses like say, skipping data? seek= and skip= are in block sizes, that may not be ideal for performance.

A smart tool should be able to write in 1 MB blocks and yet still skip 512 bytes.