Similar request has been discussed already several years ago here http://lists.busybox.net/pipermail/busybox/2005-August/049611.html and while looking file history it has been implemented, but some code refactoring have got it disappeared. Could it be done again?
Fixed in git: commit 71f6c1aa433fb2be5c3e583ca350c1f9931817a5 Author: Denys Vlasenko <vda.linux@googlemail.com> Date: Tue Nov 26 11:44:27 2013 +0100 logread: flush output. closes 6710