There is no existing method for achieving what you would want too, any 3rd party attempt would be deeply involved and complex and also could not be published as it would violate LFS forum regulations.
Modifying the car shape can yield an ingame advantage and for this reason LFS does not permit it or make it easy.
The link at Master Skinnerz that you posted was about 3D rendering. Right now, these types of modifications are not permitted by the LFS devs, and as such, you will not find information on how to do so. Kegeteys has said that he will not publish his work, nor tell anyone how to do it. AFAIK, he is the only person to have gotten a third party car into LFS.
I thought you were insinuating that it was as easy as modifying the CMX and then plopping it in a folder in LFS or something. My mistake
Depends on the car, and depends on the way LFS uses the Alpha channel on those textures. Some games use the alpha to control the light mapping, some use it for transparency. Some cars (XR cars for instance) use a tiled texture for the dash and the roof. It would look really funny to have an invisible dash :P
Also, you would only see through the roof then from in car. Outside it would appear normally.
I'd like to add something; Without a rollcage, just about no convertible streetcar would have a stiff enough chassis to be able to compete with other cars.
Even with a roll bar a convetable is rather flexable through the mid section. My miata with a 4 bar still twists the chassis as much as the suspension deflects when crawling over speed bumps at a diaginal angle. You make a roadster or convertable have acceptable chassis flex I think you will need to completly box the chassis with braces. Upper and lower systemtion front and back, a good cage and a center tunnel brace.
Besides the XRT is already ugly why make it worse by removing its top?