I can hardly blame DR5 for not giving away his trade secrets, quite to the contrary, I'm impressed he responded here at all.
I do have a theory about Rodinal and brown silver: in the FD step you have a strong solvent which completely removes tiny grains but leaves not so tiny grains behind as tiny grains. Tiny grains would normally either
- not develop at all because they are very insensitive to light
- be dissolved away by any moderately solvent developer
Now all of a sudden we have a strong re-exposure step which makes sure these tiny grains are exposed and will develop, too, and we have a more or less non-solvent developer (Rodinal) which happily develops them into tiny (and therefore brown) Silver crystals. DR5's suggestion to use a developer high in HQ (i.e. high contrast, i.e. won't develop tiny grains) and high in Sulfite (i.e. dissolves tiny grains before they develop) would certainly point in that direction.