interpolates a support glyph at the current layer. Must have a {x,y,z} formatted layer name. Designspace file path is stored in your font's lib.

Add_Support_At_Current_Layer.py

from fontTools.designspaceLib import DesignSpaceDocument, InstanceDescriptor
from ufoProcessor import DesignSpaceProcessor, ufoOperator
import tempfile
import os
import shutil
from mojo.UI import GetFile

f = CurrentFont()
file = f.lib.get("com.connordavenport.interpolateSupport.designspaceFile")
if file:
    sourcePath = file
else:
    sourcePath = GetFile()
    f.lib["com.connordavenport.interpolateSupport.designspaceFile"] = sourcePath

sourcedsp = DesignSpaceDocument.fromfile(sourcePath)

# make temp dsp with no supports
d = DesignSpaceDocument()
for a in sourcedsp.axes:
    d.addAxis(a)
for s in sourcedsp.sources:
    if not s.layerName:
        d.addSource(s)
p = f"{tempfile.NamedTemporaryFile().name}.designspace"
d.write(p)
# -----------

test = False
g = CurrentGlyph()
gname = g.name
lname = g.layer.name 
#check that we are in a support layer
if lname[0] == "{" and lname[-1] == "}":
    loc = [float(l) for l in lname[1:-1].split(", ")]
    if len(loc) != len(d.axes):
        print("layer locations != designspace axes")
    else:
        loc = {a.name:loc[i] for i,a in enumerate(d.axes)}
        doc = ufoOperator.UFOOperator(p, useVarlib=False, debug=False)
        doc.loadFonts()
        glyph = doc.makeOneGlyph(gname, loc)
        if glyph is not None:
            dest = RGlyph()
            dest.fromMathGlyph(glyph)
            dest.name = glyph.name
            g.layer[gname] = dest
            
os.remove(p)