A simple Python script that loops through your font, looks for anchors, and spits out a mark feature.

printMarkFeature.py

'''
    Print all requested anchors as a mark feature
    Requires Robofont, Robofab and the Adobe FDK
'''

# TODO: Include mkmk?

####################################################
### Edit the next settings to match your font setup.
####################################################
# Which anchors do you want to listen for?
# This dictionary defines the anchor name and the mark class.
# You can add more as you need
anchors = {
		'top': '@mark_top',
		'base': '@mark_base',
		'below': '@mark_below',
		'ogonek': '@mark_ogonek',
		'_top': '@mark_top',
		'_base': '@mark_base',
		'_below': '@mark_below',
		'_ogonek': '@mark_ogonek'
	}

# Want to give your lookup a particular name? Here’s how.
lookupname = 'marks_latin'

##################################
### From here on, no more editing!
##################################

f = CurrentFont()

# Start of the lookup code. We’ll append to this later
# These string should probably not be modified, but there is a lot of repetition
# For now, I guess appending is a lesser evil
newlist = 'feature mark {\n\tlookup ' + lookupname + ' {\n'
markclasses = ''

# In order to get AZaz alphabetic order, we take the names,
# sort, then use that as the key for the real loop
names = f.keys()
names.sort()

# Store all the names of the glyphs you need in the GDEF table
# Also store the diacritics
gdef_names = ''
gdef_diacritics = ''

# First, make sure we have a font open
try:
    f
except:
    print 'No font active. Please have a font active.'
    raise
else:
    # There is a font active. Run the code
    for n in names:
    	# Now n is a string. We need to have an object. So:
    	g = f[n] # Set the g object to be a glyph object again
    	if g.anchors:
    		base_anchor_vals = '' # Create a variable that contains all the anchors per glyph
    		mark_anchor_vals = '' # Store the diacritics ones as well

    		for a in g.anchors:
    			if a.name.startswith('_'):# If it does, this is a diacritic
    				mark_anchor_mark = anchors[a.name]
    				mark_anchor_vals += '<anchor ' + str(int(a.x)) + ' ' + str(int(a.y)) + '> ' + mark_anchor_mark
    			else:# If it doesn’t, this is a base glyph
    				base_anchor_mark = anchors[a.name]
    				base_anchor_vals += ' <anchor ' + str(int(a.x)) + ' ' + str(int(a.y)) + '> mark ' + base_anchor_mark

    		if a.name.startswith('_'):# If it does, this is a diacritic
    			gdef_diacritics += g.name + ' '# Add the name to the diacritics we touch
    			markclasses += 'markClass [' + g.name + '] ' + mark_anchor_vals + ';\n'
    		else:# If it doesn’t, this is a base glyph
    			gdef_names += g.name + ' '# Add the name to the base glyphs we touch
    			newlist += '\t\tpos base [' + g.name + ']' + base_anchor_vals + ';\n'

    newlist += '\t} ' + lookupname + ';\n} mark;\n'

    print markclasses
    print newlist

    gdef_base = '@BASE = [' + gdef_names + '];\n\n'
    gdef_marks = '@COMBINING_MARKS = [' + gdef_diacritics + '];\n\n'
    gdef = 'table GDEF {\n\tGlyphClassDef @BASE,,@COMBINING_MARKS,;\n} GDEF;'
    print gdef_base, gdef_marks, gdef

Added a simple markClasses setup as well, and made the GDEF classes available for easy insertion. The loop is the ugly part now: I could roll it all into a smarter if setup, but it’s all a bit entangled.

I’ve now consolidated the two loops into one, and made more explicit variables. That’s better!