Steadfast Slayer feat with mount

[Chad Rechkemmer]

Would the Steadfast Slayer work with my cavalier on a charge if my mount and I would both threaten with 10ft reach? The feat states if you are the only character threatening an opponent.

Steadfast Slayer (Combat)
You stand alone against creatures far larger than you.

Benefit: If you are the only character threatening an opponent and you successfully attack the opponent with a two-handed melee weapon, for each size category larger than you that opponent is, you gain a +2 bonus on your melee damage roll.

[wraithstrike]

From what I understand you can choose which square of your mount that you want to say you are in for the purpose of an attack so if you can say you are in a square that would allow you to attack before the mount gets into threaten range.

I will see if I can find a rule to support that.

[Michael Grate]

The Order of the Cockatrice's challenge bonus given extra damage to the cavalier if they are the only one threatening it

Challenge bonus wrote:

Whenever an order of the cockatrice cavalier issues a challenge, he receives a +1 morale bonus on all melee damage rolls made against the target of his challenge as long as he is the only creature threatening the target.

and there was a later errata that stated that the mount does not count against this (I can't find the exact errata but you can see the discrepancies between this site and this site)

Given that, I would say it works in the same way but expect table variation unless there is a ruling on this specific issue.