Looking for free microtonal synths and/or melodic sequencers

[jancivil]

Scala MIDI files are pitchbend, so you have an argument against a methodology of Scala as a first premise. That is somewhat suspect already.

# Can retune existing MIDI files. You can convert a standard MIDI file to be in any tuning via pitch bend commands or a MIDI Tuning Standard tuning specification.
# Can relay real-time MIDI. You play on a MIDI keyboard to a soundcard or external MIDI instrument and have the tuning changed via pitch bend commands. This way you don't need an instrument with microtuning support in order to play in a given tuning.
http://www.huygens-fokker.org/scala/

Your third sentence is in total disagreement with your first; either it's terrible and you don't want to do it or it is useful and you do.

MPE is 3 things: pitchbend (a wide range typically); pressure; and CC74 timbre. And I can demonstrate microtonality which does work using this methodology; and I was doing it prior to that albeit the wide range really does more with an instrument built for it. I've done it all my life as a guitarist, bending notes. Some instruments respond better to pitchbend than others, it is true. SampleModeling saxophones (the only one of their products I've done it with) are one which does fairly convincing bends from the pitchbend lane, it's a big part of their approach.
It is also true that not every instrument is going to be satisfying with a scala support in an instrument: what does a scala file do that disagrees fundamentally with pitch bend? Pitch bend is MIDI data, to a 14-bit resolution. A .scl alters the data before the instrument; in what way? A keyboard mapping relies on the MIDI tuning standard, if the instrument doesn't use it you use pitch bend data. I don't know what your argument really is. I don't think I'd want to try and wing a set scale with the pitch bend wheel, if that's it. But i'm very into in-between pitches or getting outside of 12tET and always have been.

There are just different approaches: with a keyboard mapping you may deal in one set scale at a time; however one may be free and use pitch bend live, or thru editing; one may start with a Scala MIDI and examine it...