Thanks for such a detailed and thoughful response.
I tried to stick to the mandate of "mild overdrive with EQ", and there just seemed too many great choices.
One more suggestion...
Compare to Southampton Pedals Ictineo Preamp/EQ
www.pedalpcb.com
I've no idea how the Drebbel would sound with Harp, it's meant to be a Bass Preamp, but it's very cool on bass with powerful EQ options (I wanted to include it in my previous post).
EMU for another powerful tone-shaper:
Amp Simulator
www.pedalpcb.com
For the reverb, you could go with simple, the Sproing, or the Sproing Deluxe and then your buddy would have tremolo, to boot.
However, depending on which PCB for the dirt you get, with the Deluxe it may take a 1590DD to house it all.
Compare to J.Rockett Boing
www.pedalpcb.com
Spring Reverb with Tremolo
www.pedalpcb.com
A more manageable combo pedal would be to select 2 x 1590N1-sized — as that will fit a 1590XX.
Once you've selected a Dirt-EQ circuit, for the additional effect...
I highly recommend checking out the FV-1 projects MODULE8 and OCTAGON
FV-1 Multi-Modulation
www.pedalpcb.com
FV-1 Digital MultiFX
www.pedalpcb.com
FV-1 Digital MultiFX
www.pedalpcb.com
The Arachnid is the same as the Module8 and the Octagon, except you (or your friend) can select the eight algorithms to be included in the pedal, via the
EEPROM BUILDER. A bespoke set of reverb, &/or delay, &/or chorus and more.
There is some overlap between the MODULE8 and the OCTAGON, so perhaps an ARACHNID with custom EEPROM is the way to go.
EEPROM-BUILDER ——————— MODULE8

—— OCTAGON

—— SPATIALIST

— — MY CHOICES FOR HARP
REV001 — Hypernova

REV002 — Glimmer

REV003 — Spring

REV004 — Blackface
REV005 — Brownface
REV006 — Spatialist Room

REV007 — Spatialist Hall

REV008 — Spatialist Chamber

REV009 — Spatialist Cavern

REV010 — Spatialist Pitchverb

REV011 — Spatialist Touchverb

PIT001 — Dual Pitch Shifter

PIT002 — Pitch Modulator

FIL001 — LoFi Machine

FIL003 — Sample Hold Filter
FIL003 — Filter Modulator

MOD001 — Chorus

MOD002 — Flanger

MOD003 — Phaser

MOD004 — Tremolo
MOD005 — Harmonic Tremolo

MOD006 — Pulsar

MOD007 — Ring Mod

DLY001 — DigiDelay
DLY002 — Modulated Delay

DLY003 — Tape Delay

DLY004 — Pitch Delay

DLY005 — Space Delay
DLY006 — Kaleidoscope

UNI001 — Unison Dual Detune
UNI002 — Unison Modulated
UNI003 — Unison Double Take
I find Flanger can fake chorus or sound phaser-ish, but not the opposite way around — not sure if the controls on the FV-1's Flanger have enough adjustment to do that, but nonetheless I'd go with Flanger for modulation. A few Reverb choices, as I imagine 'verb would be a staple — Spring natch (but Blackface or Brownface could be subbed), a good hall as counterpoint to the spring, and lastly a Spatialist PitchVerb for a one-two punch.
Tape Delay chosen for its repeats' ability to get out of the way of the clean signal. Unison double-tracker for a thickener. Dual Pitch Shifter for a more complex rich thickening of tone.
I threw Ring Mod in as a wild-card, might not sound good with Harp, though.

The other wild-card if I were to replace Ring Mod would be Sample Hold Filter.
I can imagine what harmonica sounds like through a Ring Mod, but can't imagine the Sample Hold Filter — would love to hear what that sounds like.
Again, I don't play harmonica, wish I could if only to build a cool board for it; so my choices are best-guesstimates, at best.