Code by Graham Douglas to strip glue and replace with spaces

lineparse.lua

local HLIST = node.id("hlist")
local GLUE = node.id("glue")
local GLYPH = node.id("glyph")
local WHAT = node.id("whatsit")

function lineparse(line)

      local box_order=line.glue_order
      local box_ratio=line.glue_set
      local box_sign=line.glue_sign --%0 = normal, 1 = stretching, 2 = shrinking
   -- % This loop uses a function to look for all lines of the typeset paragraph
  --  % This function scans all the items in a typeset line of the paragraph
	for n in node.traverse(line.head) do
      -- % We have seen some glue: calculate its size, draw a box
	   if n.id == GLUE then
            
           -- % glue parameters
			local gwidth=n.width --%horizontal or vertical displacement
			local gstr=n.stretch --%extra (+ve) displacement or stretch amount
			local gstrord=n.stretch_order --%factor applied to stretch amount
			local gshr=n.shrink --%extra (-ve) displacement or shrink amount
			local gshrord=n.shrink_order --%factor applied to shrink amount                 
            local width=0
    		--% This section is how TeX calculates glue settings
    		-- see 
            if box_sign==0 then --%(normal)
              	width = gwidth
             elseif box_sign==1 then --%(stretch)
             	if gstrord == box_order then
               		width = gwidth + (box_ratio * gstr)
             	else 
             		width = gwidth
             	end
             elseif box_sign == 2 then --%(shrink)
              	if gshrord == box_order then
            	  	width = gwidth - (box_ratio * gshr)
            	else
            	   	width = gwidth
            	end
             end --%if box_sign


            -- now calculate the glue's width in bp .
            if width > 0 then --% don't bother with zero width glues

                if n.subtype == 13 then 
                    print(SPACEVECTORS[51].width - width)
		            local g = node.new("glyph")
		            g.font = 51
		            g.char= 32 
                    g.width= SPACEVECTORS[51].width
                    g.lang = tex.language
                
                -- need a kern to account for any difference in (set) glue and space width
                -- to ensure vusual appearance is preserved: <glue>---> <space> + kern
                local krn = node.new("kern")
                krn.subtype=1
                krn.kern=-1*(SPACEVECTORS[51].width - width)

            	--local pdfn=node.new(WHAT, "pdf_literal")
           	 	--local wp = (width/65536)*(72/72.27)
	  			--pdfn.data=" 0.2 w 0 0 "..wp.." 5 re q 0.9 0.9 0.0 rg f  Q "
      			--pdfn.mode=0
    			--% insert into the typeset paragraph
				local np = n.prev
                local nn = n.next
                np.next=g
                g.next=krn
                krn.next=nn
                end
            end 
    	end --% if n.id == GLUE
    
         --% Insert the line bounding box
    	 local lw = line.width
            local ld = line.depth
            local lh = line.height
            local lhw = ((lw/65536)*(72/72.27))
          	local lhd = ((ld/65536)*(72/72.27))
          	local lhh = ((lh/65536)*(72/72.27))
            local ltot=lhd+lhh
            local pdfn=node.new(WHAT, "pdf_literal")
            pdfn.data=" 0.1 w 0 -"..lhd.." "..lhw.. " "..ltot.." re q 0.9 g f  Q"
      		pdfn.mode=0
            --local ln=line.head.next
            pdfn.next=line.head
            line.head=pdfn
         
	end -- for n in node.traverse
end -- end of function

parsepara = function (head)
for line in node.traverse_id(HLIST, head) do
    lineparse(line)
end
return head
end