I'm running an AT-AT event as well and I am using Boris's rules as a base. I did come up with a couple of rules updates though and I changed up the squads to closer follow the movie and use the Elite AT-AT Driver but not Rieken for obvious reasons.
After a few playtests, I came up with an additional rule to help the Rebels not get immediatley neutered. Without Deadeye their ability to damage the AT-ATs is minimal so they need the Hoth Trooper Officer to survive. Each of the 4 HTO's is set up "inside" a Golan Cannon and hise CE can be counted from 6 squares of the Golan. HTO cannot be targeted while in the Golan. Once a Golan Tower is destroyed, provided it Fails the Leia CE, the HTO can immdiately move 6 squares to jump in the trench.
Also I really liked the rule about the Pilot's coming into the fight when the Snowspeeders go down so I extended it to the Rebel Troopers on Taun Taun and the AT-AT's. When a Rauntaun goes down a Hoth Trooper replaces it on the field on a save. When an AT-AT goes down I roll a save for each of the following squad:
18 Snowtrooper with E-Web Blaster
14 Snowtrooper Commander
12 Elite AT-AT Driver
36 Elite Snowtrooper x3
11 AT-AT Driver
42 Snowtrooper x6
Special rule: Set up the E-Web. Adjacent Snowtrooper and Elite Snowtrooper may choose to replace turn and set up E-Web. At the beginning of the next round replace the Snowtrooper and Elite Snowtrooper with Snowtrooper with E-Web Blaster. If one of the two troopers is defeated before the beginning of the next round the other may activate as normal. Any damage accrued by a trooper is applied to SwEWB.
The crash survival Save at 16 can be a bit harsh so depending on the crowd a Save of 11 might be more fun for everyone.
IMPERIAL
--Hoth Invasion--
800 AT-AT Imperial Walker x4
264 AT-ST x6
54 Blizzard Scout 1
36 Elite AT-AT Driver x3
22 AT-AT Driver x2
24 Probe Droid x3
1200pts.
The AT-AT Drivers are set up as extra command crew riding in the AT-AT heads- Can combine Fire while aboard.
REBEL
--Echo Base Defense Frontline--
45 Luke's Snowspeeder*
380 Rebel Snowspeeder x10*
104 Golan Arms DF.9 Anti-Infantry Battery x4
22 Wedge Antilles*
126 Hoth Trooper with Repeating Blaster Cannon x6
19 Princess Leia, Hoth Commander
18 Rebel Leader
51 Hoth Trooper Officer x4
60 Hoth Trooper with Atgar Cannon x4
26 Rebel Officer x2
78 Rebel Trooper on Tauntaun x6
204 Elite Hoth Trooper x17
49 Hoth Trooper x7
1198pts. *Wedge, Luke HPU and Dash Rendar are in their Snowspeeders and come into play when they go down.