Well, this one may be tedious but I have a suggestion.
The 1861 tax roll for Monroe County, MS shows an Edmond T. Trewett with one slave, one pleasure carriage and one clock.He is not shown on the 1857 or 1868 rolls, the earliest and latest after 1861 that I could find.Those are available, free, at www.familysearch.org though the State Archives may have more years available.
The Archives in Jackson does have, on microfilm, the county's deed books for the war period and the decades before and after.A link to their catalog follows.At the bottom, click on "County Records on Microfilm" to see what they have for each county.
If it were me, I'd go through them (the online catalog says an index is available) to see what property Mr. Trewett owned.
With a land description in hand, you can then find the property on a county map and, in theory, have a possible location for the cemetery.The State DOT has county maps online which show identified cemeteries, in addition to the sections/township/range required to locate the property.
Here's the link, just click on the county that you need:
Topographical maps from the 1980s are also available online and, sometimes, identify cemeteries not shown on the DOT maps.Here's a link for them, click on the "Topo" button after you find your location: