Riffing on Stephen's script. Toggle whether selected glyphs are export or non-export, depending on what's happening with the first glyph selected. EDIT: just added a version that acts as a contextual menu item, for easy access.

menu-toggle_skip_export.py

from mojo.subscriber import Subscriber, registerFontOverviewSubscriber


class ToggleSkipExportMenu(Subscriber):
    
    '''
    A contextual menu item: 
    Go through all selected glyphs, and change whether they are set to export or non-export, 
    based on the opposite of the state of the first glyph.

    2023.03.21
    '''
    
    def fontOverviewWantsContextualMenuItems(self, info):
        self.f = CurrentFont()
        
        if self.f is None:
            return
        if not self.f.selectedGlyphNames:
            return
        
        myMenuItems = [
            ("Toggle Skip-Export", self.skip_export_selected_glyphs)
        ]
        
        info['itemDescriptions'].extend(myMenuItems)
    
    
    def skip_export_selected_glyphs(self, sender):
        # check to see if the public.skipExportGlyphs key is in the lib. if not, make it
        try:
            # stephen’s duplicate-clean-up step, but sandwiched in here
            self.f.lib['public.skipExportGlyphs'] = list(set(self.f.lib['public.skipExportGlyphs']))
        except:
            self.f.lib['public.skipExportGlyphs'] = []

        # starting empty lists to print out later when filled
        non_exp = []
        exp = []

        # check first glyph of selection, and plan to set everything to the opposite state of that
        if self.f.selectedGlyphNames[0] in self.f.lib['public.skipExportGlyphs']:
            make_nonExp = False
        else:
            make_nonExp = True

        # let's change states
        for g_name in self.f.selectedGlyphNames: # self.f.selection is deprecated
    
            g = self.f[g_name]
    
            if make_nonExp == True:
                if not g_name in self.f.lib['public.skipExportGlyphs']:
                    self.f.lib['public.skipExportGlyphs'].append(g_name)
                    non_exp.append(g_name)
	    
            elif make_nonExp == False:
                if g_name in self.f.lib['public.skipExportGlyphs']:
                    self.f.lib['public.skipExportGlyphs'].remove(g_name)
                    exp.append(g_name)
	    
            g.changed()
            
        # print results
        if exp != []:
            print("NEWLY SET TO EXPORT:")
            print(exp)
            print()
    
        if non_exp != []:
            print("NEWLY SET TO NON-EXPORT:")
            print(non_exp)
            print()
            
if __name__ == "__main__":    
    registerFontOverviewSubscriber(ToggleSkipExportMenu)

toggle_skip_export.py

# menuTitle: Toggle Skip Export - Selected Glyphs

'''
Go through all selected glyphs, and change whether they are set to export or non-export, 
based on the opposite of the state of the first glyph.
'''

f = CurrentFont()

# check to see if the public.skipExportGlyphs key is in the lib. if not, make it
try:
    # stephen’s duplicate-clean-up step, but sandwiched in here
    f.lib['public.skipExportGlyphs'] = list(set(f.lib['public.skipExportGlyphs']))
except:
    f.lib['public.skipExportGlyphs'] = []

# starting empty lists to print out later when filled
non_exp = []
exp = []

# check first glyph of selection, and plan to set everything to the opposite state of that
if f.selectedGlyphNames[0] in f.lib['public.skipExportGlyphs']:
    make_nonExp = False
else:
    make_nonExp = True

# let's change states
for g_name in f.selectedGlyphNames: # f.selection is deprecated
    
    g = f[g_name]
    
    if make_nonExp == True:
        if not g_name in f.lib['public.skipExportGlyphs']:
            f.lib['public.skipExportGlyphs'].append(g_name)
            non_exp.append(g_name)
	    
    elif make_nonExp == False:
        if g_name in f.lib['public.skipExportGlyphs']:
            f.lib['public.skipExportGlyphs'].remove(g_name)
            exp.append(g_name)
	    
    g.changed() # officially "updates" the glyph. g.update() is deprecated. 
    # doing the above on the "glyph" level should help it update in the UI
	
# print results
if exp != []:
    print("NEWLY SET TO EXPORT:")
    print(exp)
    
if non_exp != []:
    print("NEWLY SET TO NON-EXPORT:")
    print(non_exp)