Well i was thinking why subtitle are not implemented , well no time i think it's the why.. , but subtitle should be easy to add
i mean subtitle composer 0.5.3 it's very nice on this but they use mencoder....

but with ffmpeg should be easy with this

ffmpeg -h | grep subtitle

-newsubtitle add a new subtitle stream to the current output stream

i know it's a stupid stuff but... reading manual is boring so i have written this.