Mp3tag 1.2.0
It's possible to do this without using AviSynth at all, but it's a bit of a hassle. You need WaveWizard/Foobar2000 or WaveWizard/the FLAC Frontend (which is automatically installed when installing FLAC 1.2.0). [discussion].
Mp3tag 1.2.0
Get Foobar2000 v0.9 and the AviSynth input plugin "foo_input_avs.dll". Put the input plugin in the foobar2000\components folder. After installing FLAC 1.2.0, create your AviSynth script. How to do that is explained in the previous sections, and won't be repeated here. Open your script in Foobar2000 using the File tab. Right-click on the script, select "Convert" and select "Convert to": 041b061a72