MP3 AudioFormatWriter

Any plans to implement this ?

AudioFormatWriter* MP3AudioFormat::createWriterFor (OutputStream*, double /*sampleRateToUse*/,
                                                    unsigned int /*numberOfChannels*/, int /*bitsPerSample*/,
                                                    const StringPairArray& /*metadataValues*/, int /*qualityOptionIndex*/)
{
    return nullptr;
}

or there is no chance and it will remain read-only

No, no plans to do that currently.