You need to be able to tell your chord generator what is actually possible with the instrument you're writing for - you don't want impossible keyboard stretches, out-of-range notes or multiple simultaneous notes on the same string.
There is an auto-harmonizer in Athanasius Kircher's Musurgia Universalis (1655), using sliding rods with chord pitches on them. Intended for voices, but the problem is that as you get lower the vocal lines have to leap around more and more to fit the rules of harmony. The bass has a really tough time with this one, though it sounds fine:
T:Tristrophon hendecasyllabum Armenicum
T:Melothesia Armenica secundi toni
S:Athanasius Kircher, Musurgia Universalis (1650), lib.VIII p.136
N:If I understand the Latin correctly he got this from the
N:Jesuits and harmonized it using his mechanical rules.
N:The second G in voice 3 bar 1 is E natural in my notes;
N:dunno if I transcribed it wrong or Kircher had a typo.
M:8/2 % C|
Q:1/2=60 % guess
W: Garrachat parraz nuruche smidaz luhs
W: Ansdieziet ariechagan luhs dzachia huchus
W: Pirchieziet puguriz sesa puta pirchel.
W: Splendor gloriae renovat mentis lumen
W: Increati itaque solis lux orire huic animae
W: O Redemptor Universi hanc festina redimere.
V:3 transpose -12
V:4 transpose -12
[V:1] d2 dd B2 d d2 cAG c2d2 |B2dc B2c B2A AB A2 A2|G2GG A2A B2B GG ^F2G2 ||
[V:2] G2^FG G2=F F2_EDG F2F2 |D2DE G2A G2F EG E2^F2|D2DD F2F G2F _ED D2D2 ||
[V:3] B2 AB G2 A D2 GFB A2B2 |d2Bc d2f d2d ^cd2 ^cd2|B2BB c2c _e2d cB A2G2 ||
[V:4] G,2DG _E2 D B,2CDE F2B,2|B2BA G2F G2D A,G, A,2D2|G2GG F2F _E2B, CG, D2G,2||
I nominate that as one of the first-ever pieces of "world music". Kircher got tunes from Jesuit missionaries and harmonized them himself. (There's another one using an Arabic Christian chant where Western-style harmonization was an even more improbable project). Terrific tune and I don't see any reason why it couldn't work for concertina band, though.