I remember PET (photoinduced electron transfer) works by the Mariano group in which N-dimethylallyl pyridinium perchlorate underwent intramolecular radical cyclizations upon uv irradiation and in fact, I've seen N-methyl chinolinium salts undergo dimerization under these conditions.
Imagine that the N-methylpyridinium cation is irradiated at $\lambda$ = 300 nm. The resulting $S_1$ state oxidizes cyanide to a cyanogen radical.
The N-methylpyridinium cation is thereby reduced to a pyridyl radical.
Radical dimerization furnishes a 4-4'-dihydrobiaryl (see drawing).
Aromatization to paraquat requires the loss of two hydrogen atoms and two electrons; the latter are being used to regenerate cyanide.
The mechanism is a bit speculative but consistent and reflects the catalytic use of cyanide.
Note: I would have liked to add a sketch for the mechanism but BKChem died on me several time while doing so.