The way I’ve solved the problem before 9.4 is to use a command called 'pv' (pipe view). Normally this command is useful for seeing the rate of data flow in a pipe, but it also does have a rate limiting capacity. The trick for me was running the output of pg_basebackup through pv (emulates having a slow disk) without having to have double the storage when building a new slave.
First, 'pg_basebackup' to standard out in the tar format. Then pipe that to 'pv' to quietly do rate limiting. Then pipe that to 'tar' to lay it out in a directory format. Tar will dump everything into the current directory, but transform will give you the effect of having selected a directory in the initial command.
The finished product looks something like:
pg_basebackup -U postgres -D - -F t -x -vP | pv -q --rate-limit 100m | tar -xf - --transform='s`^`./pgsql-data-backup/`'