Ι've been doing something like that since 2012 but differently.
I split a Bluechart for Mapsource, install it and then I choose all maps with Map tool.
I open the Bluechart with Gmaptool and go to Options > Info
I compare the the sub-maps in Mapsource map list with Gmaptool Info list and delete from Mapsource map list all the sub maps that do not have the "T" (Transparent) in Gmapstool Info list.
From Mapsource I sent to device all the remaining maps.
This is a Transparent map with only the contours in it.
Then I copy the original Bluechart on the device.
Now on device (3490) I have the Bluechart map (solid) with sea colors and no contours (because contours are part of Fishing Mode in a Bluechart).
I also have the Transparent contour map that I've sent with Mapsource.
If I check them together, I have a full Bluechart.
If I check the Transparent contour map with a Road map I can see the contour lines and the road map.
Bluechart original on 3490, checked alone
Bluechart original on 3490, checked with Mapsource's transparent contours
Mapsource's transparent contours, checked alone
Mapsource's transparent contours, checked with City Navigator Europe NTU
3490 map list
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