Jude, a fixed scale of just intonation can be approximated arbitrarily close by an EDO. Your 21 note scale, which I assume to be C C# Db D D# Eb etc. where sharp and flat alter by 25/24 and the white notes are 1/1, 9/8, 5/4, 4/3 etc. could be defined in 72edo so that the white note scale is defined in steps as (0,)12,11,7,12,11,12,7 and sharp and flat defined as 4 steps. If you want better accuracy in the playback, you can use a larger EDO, such as 205EDO.
Any accidentals and any EDOs can already be used and defined in Finale, with correct playback (polyphonic, without pitchbend). For example, I use 31EDO notation and playback in Finale for ¼-comma meantone; they’re virtually identical. However, a re-tunable instrument has to be used, with tuning scripts such as scala, and one is limited to 128-note tunings on a single midi channel. Obviously this is not enough for larger EDOs so Dorico’s proposed VST3 note expression method is extremely welcome.
However, music with fixed scales in Just Intonation is a special case, whereas in principle JI has an infinite number of pitches and the notation (Johnston, Sabat-v.Schweinitz, etc.) likewise an infinite number of accidental combinations. A Johston symbol font is unlikely to include all the combined accidentals needed for notating a string quartet by Johnston himself. There may be a symbol for the seventh harmonic of the seventh harmonic (77) but what if the composer decides need that flattened by 25/24 and 77b is missing in the font? For notating JI, it would be ideal to be able to combine accidentals freely and have the combinations play back correctly. No notation software is able to do this - “cumulative” accidentals -, although Finale and Mus2 do handle non-equal tunings with microtonal accidentals.
Finale is already capable of notating JI the way Jude describes above (”…those adjustments can be connected to the use of custom accidentals (or any accidental)”. This is rather awkward, though, because in this case the accidentals are manually inserted text symbols, and pitchbends are used for playback in addition to script-tuning (for the unequal white note scale). But the accidentals do send out the pitchbend messages themselves. Dorico promises less if I understood correctly.
As a microtonal composer who often works in JI, I’m disappointed by Dorico’s decision to leave out full support for this important area of 20th and 21th Century music, apparently the only reason being that it’s trickier to program than EDO’s - only few of which are as popular and wide-spread among microtonalists as JI.