A long time ago, a patch was submitted to add the fstrim applet to busybox: http://lists.busybox.net/pipermail/busybox/2012-March/077571.html (note: not my patch!) It looks like a prior version of this patch (http://lists.busybox.net/pipermail/busybox/2013-February/078911.html) received some minor review comments, which was addressed in the updated patch(?), but the patch didn't progress any further. With the proliferation of flash based storage benefiting from fstrim, this would be a helpful applet to include in busybox.
Thanks for the reminder, applied as 1d39e33d46f32862da2ec1c394401239515206b8