Hmm, technically I could keep the linear phase, low ringing and variable phase delay if I just abused FFT.
A low order FFT (like 1024-point or less) could be used too with Lagrange interpolation between multiple runs offset by a single sample.

I think I'll make that into actual v4. The next step is a combo with an FFT-based linear phase graphic equalizer with very high number of bands and any kind of time delay at any frequency.
The cost is the increase in total delay from 180 to e.g. 515 samples and major increase in CPU utilization. (But I can get most of that back by using a fast FFT library.)
Plus it won't be in Faust anymore, as it is definitely not suited to FFT-based effects.

Edit: Bah, never mind, trigonometric interpolation after FFT is way better: http://www.dsprelated.com/showcode/18.php