I remember wanting the same but.....no ...impossible

*the only one i know is "
StadiumDirtPub2x1S.dds"
IF is the big white sign from tm and those little on the floor around dirt road (not the round one)....
you
can't change it unless you ask people to change their program.
This sign dosen't come in the path of the game textures, so not in the mod pack.
the big sign on top of dirt turn can be change but only you and the people who change it will see it.For example, here's the name to the file : "
StaticAd2x1A" (
for the big panel)
C:\Program Files\TmUnitedForever\GameData\Interface\Advertising
**************************************
The tm signs found on the border of the dirt can be change
(in the mod)"
StadiumCircuitScreenMask" does the trick, but when you do this, you also change
those one 
OFC, you can try "StadiumWarpPub8X1S" and "StadiumWarpPub8X1SN" even create a "
StadiumWarpPub8X1D.dds"
That'S how it looks when all the "ADD" file from
C:\Program Files\TmUnitedForever\GameData\Interface\Advertising is changed .