You could just make one from photoshop or something then import it into FCP X. Then drag it to the timeline over the top of your main storyline clips and extend it for as long as needed in time. Select the grid clip then go into inspector and set the opacity to make it as transparent as you need for a reference. Obviously it's not a FCP X thing, and I have no clue if such a feature is lurking in there somewhere ... but something like this would get you by. Best wishes.
I just posted this on Apple's FCP 'Enhancement Requests' page.
In addition to the Show Title/Action Safe region, could we have a scaleable grid, or graticule, as an overlay, or pull-down View option, for precisely aligning scaled clips and stills?
There is a free downloadable effect to do this, which I have used but it has to be continually enabled/disabled in the Timeline, or it will end up in the exported material.
I find this 'grid' method faster than tweaking sliders in the Video Inspector.