Shoehorning in a prestige class

I commented the other day that archetypes make a fantastic replacement for prestige classes in terms of mechanical diversification.  So now I've spent most of the morning thinking about whether prestige classes can even work in an E6 environment, given that at best you're looking at precisely one prestige level, and the mechanical/flavor essence of the class typically doesn't emerge until you're midway through its progression.

I've also been meaning to toy around with a decent-charisma rogue build.  I'll take aim at both birds here, though I don't think this is a playable character in our current campaign, given that the prestige class is necessarily evil and built around an exotic weapon, which seems silly.

Class: Rogue 5 (scout archetype) / Red Mantis Assassin 1
Race: Human

Str: 12 [2]
Dex: 20 (17 + 1 level + 2 human) [13]
Con: 12 [2]
Int: 14 [5]
Wis: 8 [-2]
Cha: 14 [5]

H1: Alertness
L1: Exotic Weapon Prof (Sawtooth Sabre)
L3: Two Weapon Fighting
L5:  Weapon Focus (Sawtooth Sabre)
E7:  Weapon Finesse
Bonus:  Weapon Specialization (Sawtooth Sabre) (from prestige class)
Bonus:  Dodge
RP1:  Offensive Defense (+4 dodge AC bonus for one round after dealing sneak attack damage)
RP2:  Combat Trick (bonus combat feat)

Abilities:  Sneak attack +4d6, Trap Sense +1, Scout's Charge, Evasion, Trapfinding, first level spells
Equipment: Studded Leather +1, Deflection +1, 2x Sawtooth Sabres +1

To hit:
3 BAB
5 dex
1 Weapon Focus
1 weapon enhancement
-2 two weapon fighting
= +8

Damage:
1 strength
1 weapon enhancement
2 weapon spec
= +4

AC:
10 base
5 dex
3 armor
1 armor enhancement
1 deflection
1 dodge
= AC 21

Normal Attack:
+8 [1d8+4]
+8 [1d8+4]
AC 21

Flanking Attack:
+10 [1d8 + 4d6 + 4]
+10 [1d8 + 4d6 + 4]
AC 25 (assuming a hit) or 21

Charge Attack: 
+10 [1d8 + 4d6 + 4]
AC 23 (assuming a hit) or 19

This build is focused on getting into flanking position and using 4d6 sneak attack dice to do the bulk of his damage, plus boosting his AC for survivability from the rogue trick.  Mobility is probably the next feat to take at E8.  If flanking isn't possible, the scout archetype allows him to get one sneak attack on a charge, which helps his poor hit value and still potentially puts him ahead of base in terms of AC.  And if neither flanking nor charge is possible, he has first level illusion/transmutation spells (two to cast, don't know how many known - that chart is missing from the SRD), so:

Color Spray: Stun targets for one round, which denies them dex, which opens up sneak attack
Vanish:  Invisibility for 1 round, which would set up a next-round sneak attack (unless the one round ticks off at the start of the next round instead of the end?  Only having one caster level makes this tough.)

Either way, it's a more involved melee style that requires mobility and adaptabiltity rather than standing still and hacking away, it replaces some of the stealthy/skillsy stuff we lost when we killed our previous rogue, and its primary weakness (requiring lots of feats to establish) is mitigated by the E6 format where feats should come easily.