Dayum !
#!/bin/bash
gmt begin test png
# generate a simple sinusoïd
vecsta=$(gmt math -Q -1 PI MUL =)
vecsto=$(gmt math -Q 1 PI MUL =)
vecste=$(gmt math -Q 1 32 DIV =)
gmt math -T$vecsta/$vecsto/$vecste T -C1 SIN = sin.txt
# use @Esteban82's magic
gmt grdmath -R-3.5/3.5/-1.5/1.5 -I0.01 Y = grd.nc
gmt grdmath -R-3.2/3.2/-3.2/3.2 -I0.1 X Y ADD NORM = grd_out.nc
gmt grdedit grd_out.nc -Er
# define outside/inside colours
gmt makecpt -Cgeo -T-3/3 -H > color_fg.cpt
gmt makecpt -Cseafloor -I -T0/1 -H > color_bg.cpt
# fill outside using inversed clip of function (forces a closed polygon similar to plot -L)
gmt clip sin.txt -N
gmt grdimage grd_out.nc -Ccolor_bg.cpt -t25
gmt clip -C
# fill inside using clip
gmt clip sin.txt
gmt grdimage grd.nc -Ccolor_fg.cpt -t25
gmt clip -C
# redraw function for nicer result
gmt plot sin.txt -Wthick,black
gmt end show
This is kind of neat 
