The following method works in TS2009SP3 and, suitably backdated, earlier versions. Your mileage may differ in later versions, and it's a bit of a cheat since it adds mesh rather than just reskinning per se.
(1) First, you need to make the existing numbers vanish since you want your new ones somewhere else. If you don't re-color them, the contents of the 3-6 images in the main folder will show the traditional "456" or "123456" etc. (because that mesh will no longer be "Default," see below), usually in some strange color or font if the creator was lazy (*guilty!*) So what you do is replace the .tga files with 32x32 images in your desired color at the old number locations, including stripes and such, remembering that they'll be rotated on the other side. Unfortunately, *these* images have no alpha channel - it would be nice if you could replace them with .tga that's all background, but you can't.
(2) Next, create new numberboards and side numbers in GMax or Blender. Export them to the main folder and make them part of the rendering by including this new mesh in the mesh table. Move them around until they're where you want them, about .00625m out from the surface. (This is the cheat - you have to make content for the method to work.) If you want two different font styles or colors (or more) be sure to set them up that way; put your new font .tga files with appropriate names, *with* alpha channel, in the designated fonts folder. (Leaving out a lot of detail here, such as how to create fonts, increasing the font count if required, etc.!)
(3) The final trick is that apparently only the first mesh in the mesh-table (conventionally named "Default") is processed in-game for changeable road numbers, so your add-on mesh with only the number boards, side numbers, etc. will have to be "Default" and the original creator's mesh that you're reskinning will have to be something else ("Default1" for example). Make those changes in the config and, lo, you will have changeable road numbers in the locations, sizes and colors you want! (This, by the way, is why you have to re-color the digit .tgas in the main folder: the old numbers are part of the renamed "Default1" mesh, so will display whether or not changeable road numbers are designated in Surveyor - because they're no longer processed as road numbers.)
(I have a witness that the above wacky procedure works, but can't produce the witness because we're partners in crime, as it were.)