Did it !
added a few merging option (not to merge with the original but with a copy of the gmic result )
very last merging option has a unusual name "Skip Merging" but i hope a clear meaning
(well was not really needed you get the same at opacity 0 with all merging mode...anyway
HERE the code (if you want try, it will not conflict with original David filter)
#######################################################
#@gimp Stained glass 2 : gimp_stained_glass2, gimp_stained_glass2_preview(0)
#@gimp : Edges = float(20,0,100)
#@gimp : Shading = float(0.1,0,0.5)
#@gimp : Thin separators = bool(1)
#@gimp : Equalize = bool(1)
#@gimp : sep = separator()
#@gimp : Merge = choice( "Color Burn ","Grain Merge","Hard Light","Soft Light","Overlay","Multiply","Skip Merging")
#@gimp : Opacity = float(1,0,1)
#@gimp : sep = separator(), Preview type = choice("Full","Forward horizontal","Forward vertical","Backward horizontal","Backward vertical")
#@gimp : sep = separator(), note = note("<small>Author : <i>David Tschumperlé</i>. Latest update : <i>2011/03/18</i>.</small>")
gimp_stained_glass2 :
-repeat @# -l[$>] -split_opacity -l[0]
-stained_glass $1,$2,$3
-n 0,255
-if $4 -equalize[-1] 256 -endif
-if {$5==0} --gimp_do_nothing -gimp_compose_colorburn $6
-elif {$5==1} --gimp_do_nothing -gimp_compose_grainmerge $6
-elif {$5==2} --gimp_do_nothing -gimp_compose_hardlight $6
-elif {$5==3} --gimp_do_nothing -gimp_compose_softlight $6
-elif {$5==4} --gimp_do_nothing -gimp_compose_overlight $6
-elif {$5==5} --gimp_do_nothing -gimp_compose_multiply $6
-elif {$5==6} -gimp_do_nothing -endif
-endl -append c -endl -done
gimp_stained_glass2_preview :
-gimp_split_preview "-gimp_stained_glass2 ${1--2}",$-1
########################################################
Merging may intensify the colors, how depend from the option you chose (i.e "multiply will also darken) and from Opacity value...max at 1, nothing at 0,
PS
@willy
glad you like feel free to use it