Mark Dunder
Mongoose
IF you START to allow a missile character to threaten people then you open yourself to a whole host of problems.
Same to you, I specifically said the archer may not threaten. If this also means you cannot flank your opponent, then so be it.
Assuming the yaggite has it correctly, unless you are directly on opposites sides of the opponent, you cannot "flank" them.
My supposition puts your target in such a position, that all sides are engaged except the one through which the archer has a clear line of sight, then and only then, can you give the archer a flanking bonus. This is an indirect bonus, given because of the archers position. The opponent may not be "aware" of his danger, and due to the assumed distance of the archer, is not "directly" engaged (threatened), by the archer. Thus the ally opposite the archer (relatively speaking) does not get a flanking bonus. As soon as the archer scores a hit, allow the target to become aware of the "threat," future flanking bonuses from that archer are lost.
This allows a small "window of opportunity" for the archer to "flank" a target.
I assumed all along that this is an "option" that allows a flanking attack by the archer. You may allow this in your game or not. I don't think it will play a significant threat to the melee, but may allow the archer to subtly influence the melee. This may give more options to those campaigns where Hyrkanians, with their incredible bows, play a greater part.
Perhaps "flanking a melee" could be a Feat available to Hyrkanians or Archers at higher levels. In fact, that's what I would do. Anyone have a suggestion as to what level and name you could give this Feat?