Vern, if I understand you correctly you want to "fake" a map projection. Assume you accept the UTM coordinate system as an ordinary Cartesian system with units in meters (which it is), you only have to scale your pixels of the raster image to meters. You can do that either by creating your own World file or your own TransDEM .trf file. Both are similar as they are define affine transformations. An ESRI world file would be regarded as pre-georef in TransDEM, while the native .trf file is post-georef, obviously.
This is a sample .trf file:
Code:
Transformation=2DAffine+UTMZone+Aux
Punkt0.x=0.000000000000
Punkt0.y=0.000000000000
Punkt0_t.x=615857.000000000000
Punkt0_t.y=5942414.000000000000
Alpha=0.000000000000
Skalierung.x=80.000000000000
Skalierung.y=80.000000000000
Scherung=0.000000000000
UTMZone=32U
Groesse.x=96000.000000000000
Groesse.y=88000.000000000000
png=1
"Punkt0_t" is SW corner in UTM coordinates. Could be 0 in your case. "Skalierung" (scaling) is meters per pixel of your image. "Groesse" (size or dimension) is the total size of the associated image in meters. If present, indicated by "+Aux" in "Transformation", it will supersede any "Skalierung".