Thanks so much!

Also, meanwhile, I figured how to do it with NeroAacEnc via pipe and ffmpeg. For those who are interested:

ffmpeg -i input.ac3 -f wav -ac 2 - | neroAacEnc -q 0.4 -if - -ignorelength -of output.mp4
I usually choose quality 0.4 for Nero AAC encoder.

However, the direct usage of the libaacplus seems just more elegant. But I don't know if the quality is on par with the Nero encoder. Would really be interested which is superior, Nero or the libaacplus.