Do you think it would be possible to use this tool to create a full C frontend (including preprocessor)? I imagine the difficult part would be passing preprocessed text (including generated source e.g. pasted tokens, and expansion locations) into the C parser.

The preprocessor would likely have to be a separate ad-hoc step, but it is possible that langcc could be used to generate a full C frontend if one is willing to run two passes (needed to deal with ambiguous cases such as "x * y", which can be either a multiplication expression or a variable declaration, depending on the contents of the symbol table).