For a few car projects that I have in mind I need to be able to shape aluminium sheet into 3 dimensional shapes and I wondered if it would be realistic to follow the usual process of making a plug and then take a carbon fiber or a very thick fiberglass cloth mold from that.

If the mold mating faces were thick and had sufficent support (the back side of the mold gussetted, braced, boxed and any internal gaps filled with dense PU foam), does this sound feasable?
For single direction bends I think it would work well, but I have some concerns where there would be definite creases and compound curves required in the part.

Actual sizes of most pieces would be quite small, maybe 200mm x 200mm at this stage. And material would be a typical weldable aluminium (probably 6061) and 2mm thick.

Yeh mate, it can be done. Did exactly this for forming an aluminium insert to go in the recess of a carbon fibre 57 dash that I made. 

Cheers for the input and conformation.
Definitely something to get sorted now I know that it should work.