Elkhorne,
One solution might be to brush lamp black (soot) onto the stock, particularly into the nooks and crannies of the carving. Then rub it back to get the lamp black off the outer surfaces, leaving it only in the incised lines. Do this prior to the last 3 or so coats of varnish. I have not tried this yet but have been told it is a good solution.
I also think that using a V gouge to do the incise carving might leave you with a rounded bottom. That seems to be what I get when I have tried it. I seem to remember a tutorial here about sharpening V gouges that presents a technique that will mitigate the rounded bottom.
I have not tried, but will soon, either stabbing it in and then beveling the other side with a knife, or using a knife making two passes to create a "V" shaped incised line. I'm told if you do it right, is leaves a nice sharp V incised line.
Cheers,
Norm