The patch below provides exactly that to the MP3 muxer. A XING header
containing

   * the numer of frames,
   * the size, and
   * a TOC

is generated.

It's based on an idea by Anton Khirnov (restricted to the number of
frames) found at

   http://patches.ffmpeg.org/patch/1891/

The TOC is generated as found in lame's "VbrTag.c".

According to my tests the following reproduces the number of frames, the
size and the TOC in "c.mp3" from "b.mp3" (except a shift due to shorter
XING header generated by FFmpeg):

   lame -V2 a.wav b.mp3
   ffmpeg -i b.mp3 -acodec copy -y c.mp3