As someone notes on the bug, if you were rolling your own, there are some other things you could do--return a [][]byte that's a pointer to its internal buffer, only usable until the next row is read.

Making a version of encoding/csv that retains most of its features (custom delimiters, handling backslashes and quoting and \r) but streams like that would be a fun open source project for someone who likes Making Things Go Fast.