Masking Ocean areas in rotated Cartesian coordinates?

I have an oblique rectangular domain, where I would like to plot a 2D data on top of topography.

I am playing with the domain in a Cartesian coordinate system:


What I am doing is finding out all Ocean areas in the topography, and try to mask them out in plotting the data, as seen in the bottom in the figure below. My approach is creating a grd file with NaNs for Ocean areas, then blending it with the data.grd:
gmt grdblend temp.grd coast_mask_extlarge.grd -Cu -I0.02/0.02 -Gtemp2grd
which, however, totally messed up my colormap.
With coast command, we can select this Ocean area (the bottom red part) and fill in some color, but how to do it in this Cartesian plot?

Thank you!

I have a workaround now, perhaps this is the correct way.
The idea is to create another grid file with NaNs and some certain number representing ocean. Then plotting this grid with -Q flag to visualize crust part, while giving some color to ocean.

gmt makecpt -Clightblue
gmt grdimage mask.grd -C -Q

I haven’t tried to use GMT to create such mask.grid from topography, instead I created a binary using Python, then convert it to mask.grid.