hI SamT,
if you really need to create a WKT, you could create a grid file for you region of interest with a constant geoid elevation of -3.155 m (this means your shifted elevations will be 3.155 m greater then elipsoidal elevations). See fisrt screen copy. I exported this grid as a Geotiff Test.tif file.
Then in MS in Reference Settings use Tools button to add the geoid file to your CRS by navigating to the location of test.tif and in Import Geoid window accept it as in 2nd screen capture.
You will now have a test.tif geoid file in the Metashape Geoids file folder. And now placing points in your custom CRS, their elevations will be 3.155 m higher then elipsoidal heights... see 3rd screen capture
The resulting COMPD CRS is:
COMPD_CS["GDA94 / Custom Transverse Mercator + Vertical Offset",
PROJCS["GDA94 / Custom Transverse Mercator",
GEOGCS["GDA94",
DATUM["Geocentric Datum of Australia 1994",
SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],
TOWGS84[0,0,0,0,0,0,0],AUTHORITY["EPSG","6283"]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9102"]],AUTHORITY["EPSG","4283"]],
PROJECTION["Transverse_Mercator",AUTHORITY["EPSG","9807"]],
PARAMETER["latitude_of_origin",0],
PARAMETER["central_meridian",120.95],
PARAMETER["scale_factor",0.999879],
PARAMETER["false_easting",446901.042],
PARAMETER["false_northing",2879773.273],
UNIT["metre",1,AUTHORITY["EPSG","9001"]]],
VERT_CS["Fixed Offset",
VERT_DATUM["Offset Value 3.155 m",2005],
UNIT["metre",1,AUTHORITY["EPSG","9001"]]]]
Hope maybe can be helpful,
PS. the extent of the Test.tif file is probably greater then your region extent, so you can crop it to fit your area of interest and then add it into your CRS as explained above...