If you want curved cell edges (not sure that would often be important physically, given BOUT++ codes are usually low-order accurate so small errors are probably not a big deal, but maybe it'd be a useful demonstration of capability for testing some code coupling approaches) it might not be too hard to add some support from hypnotoad. The poloidal cell faces I think might as well stay as straight lines joining the corners (it would be more work to add more detail here, beyond the single point in the 'centre' of the poloidal face, which is already written out as Rxy_YLOW, Zxy_YLOW). However, the radial cell faces could be upgraded to be curves. I don't know what information exactly would be needed, but for example if one wanted to add an array of n points with either uniform spacing in poloidal distance or spacing consistent with the hypnotoad grid points (which might be smoother in some sense) I think that is pretty doable. hypnotoad already has interpolation functions along flux surface contours which could translate some poloidal index-space or arc-length coordinates into (R,Z) values to be written into the grid file.
If you want curved cell edges (not sure that would often be important physically, given BOUT++ codes are usually low-order accurate so small errors are probably not a big deal, but maybe it'd be a useful demonstration of capability for testing some code coupling approaches) it might not be too hard to add some support from hypnotoad. The poloidal cell faces I think might as well stay as straight lines joining the corners (it would be more work to add more detail here, beyond the single point in the 'centre' of the poloidal face, which is already written out as Rxy_YLOW, Zxy_YLOW). However, the radial cell faces could be upgraded to be curves. I don't know what information exactly would be needed, but for example if one wanted to add an array of n points with either uniform spacing in poloidal distance or spacing consistent with the hypnotoad grid points (which might be smoother in some sense) I think that is pretty doable. hypnotoad already has interpolation functions along flux surface contours which could translate some poloidal index-space or arc-length coordinates into (R,Z) values to be written into the grid file.