You can already do that easily with GmapTool.
I have done it several times and it has worked perfectly for me.
Split the file to separate .img files
Files > Details - look for any .img file containing the word "Bitmaps" - see example below (last line) "Bitmaps 377, size 8461066 (4)".
Remove these .img files from the combination and reassemble the gmapsupp.img (or whatever you want to name it).
Raster images gone!
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File: 08237056.img, length 9068544
Header: 10.07.2011 02:39:18, DSKIMG, XOR 00, V 0.00, Ms 0
Mapset: P0000031
fat: 1000h - 1200h - 2000h, block 8192
maps: 1, sub-files 1

Sub-file fat length
P0000031 GMP 1200h 9056979
GMP: TRE RGN LBL
map 7db000 (8237056)
PID 1, FID 2028, region 5,27
date 01.03.2010 09:02:25
priority 28, parameters 1 6 31 19, transparent
levels [17,18,19,20,21,22,23,24], zoom [87,6,5,4,3,2,1,0]
N: -33.202314, S: -36.140342, W: 170.357652, E: 173.565273
GARMIN G-Map
Copyright 1995-2010 by GARMIN Corporation.
CP 1252, Western European
Bitmaps 377, size 8461066 (4)
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