SMuFL uses different characters. You need to find a spot to add this to the script, which should work in RGP as well as JW.

local diamond_head = 79 -- non-SMuFL character
local font_info = finale.FCFontInfo()
font_info:LoadFontPrefs(finale.FONTPREF_MUSIC)
-- Finale version 27+ and RGPLua suitable?
if finenv.RawFinaleVersion >= 10027 and font_info.IsSMuFLFont then
    diamond_head = 0xe0e1   -- SMuFL version of char
end

Of course! Why didn’t I see that?

I am absolutely in awe of you folks who can not only conceive of that, but make it work. Were I wearing a hat, it would be off to you.

Although he uses it differently, I borrowed those elements from Nick Mazuk's script "pitch_transform_harmonics_fourth.lua" - part of his FinaleLua script set. (https://www.finalelua.com)

That was the guidance I was looking for in terms of what to insert into Nick's script. However, I've not been able to get it to work (I tried inserting the additional code into the script and it is not providing a harmonic character, just the large O as before).

Here is the code for the .lua script (this is the one I've been using, which uses a perfect fourth). Any suggestions where to place the additional code? Thanks. 

function plugindef()
    finaleplugin.RequireSelection = true
    finaleplugin.Author = "Nick Mazuk"
    finaleplugin.Copyright = "©2020 Nick Mazuk"
    finaleplugin.AuthorURL = "https://nickmazuk.com"
    finaleplugin.Version = "0.1"
    finaleplugin.Date = "3/29/2020"
    return "String Harmonics 4th - Sounding Pitch", "String Harmonics 4th - Sounding Pitch", "Takes a sounding pitch, then creates the artificial harmonic that would produce that pitch"
end

function changeoctave(pitchstring, n)
    pitchstring.LuaString = pitchstring.LuaString:sub(1, -2) .. (tonumber(string.sub(pitchstring.LuaString, -1)) + n)
    return pitchstring
end

function updiatonicfourth(pitchstring)
    local letters = "ABCDEFGABCDEFG"
    local notenamepos = letters:find(pitchstring.LuaString:sub(1,1))
    local newnote = letters:sub(notenamepos + 3, notenamepos + 3)
    pitchstring.LuaString = newnote .. pitchstring.LuaString:sub(2)

    -- transposes everything an octave higher if necessary
    if(notenamepos >= 7) or notenamepos <= 2 then
        pitchstring = changeoctave(pitchstring, 1)
    end
    return pitchstring
end

function deletecirclearticulation(entry)
    local artics = entry:CreateArticulations()
        for a in eachbackwards(artics) do
            local defs = a:CreateArticulationDef()
            print(defs:GetAboveSymbolChar())
            if defs:GetAboveSymbolChar() == 111 then
                a:DeleteData()
            end
        end
end

function main() 
    for entry in eachentrysaved(finenv.Region()) do
        -- only process 1-note entries
        if (entry.Count == 1) then 
            deletecirclearticulation(entry)
            
            local note = entry:CalcLowestNote(nil)
            local pitchstring = finale.FCString()
            local measure = entry:GetMeasure()
            measureobject = finale.FCMeasure()
            measureobject:Load(measure)
            local keysig = measureobject:GetKeySignature()
            note:GetString(pitchstring, keysig, false, false)
            pitchstring = changeoctave(pitchstring, -2)
            note:SetString(pitchstring, keysig, false)
            
            local newnote = entry:AddNewNote()
            newnote.Tie = note.Tie
        
            -- makes the new note a diatonic 4th higher with the same accidentals
            newnote:SetString(updiatonicfourth(pitchstring), keysig, false)
        
            -- fixes any errors from the diatonic transposition
            if(newnote:CalcMIDIKey() - note:CalcMIDIKey() ~= 5) then
                local error = newnote:CalcMIDIKey() - note:CalcMIDIKey() - 5
                newnote.RaiseLower = newnote.RaiseLower - error
            end
        
            -- create a diamond character that is 110% of the default size
            local notehead = finale.FCNoteheadMod()
            notehead:EraseAt(newnote)
            notehead.CustomChar = 79
            notehead.Resize = 110
            if(entry:GetDuration() == 4096) then
                if(note:CalcStaffPosition() >= -5) then
                    notehead.HorizontalPos = 5
                else
                    notehead.HorizontalPos = -5
                end
            end
            notehead:SaveAt(newnote)
        end
    end
end

main()

It will be much simpler to get his revised version of that script from (https://www.finalelua.com)

Well, sure. I reached out to him yesterday so hoping to hear something soon. Will update the group when I do. Not mission-critical but definitely can be a time saver if you’re writing a lot of string harmonics (as I am currently). 

Just heard from him. He's tied up for the next few weeks but should have a fix out at some point.