Errata v5.1 pg 22 wrote:
Page: 251 (Errata)
Tower Shield
Add the following after Tier III Shield Slam; shown in bold
Tier III Shield Slam; allowing the wielder of this shield to attack as if his tower shield was a Large Shield
So Speed: 5, Damage: d8
As long as you combine another technique with Shield Slam (which limits the options a bit), you already can. Guarded Charge is in. Penetrate Guard is out (being Advanced).
All of the options make you lose your Av bonus, precluding its best feature.
With a Guarded Charge/Shield Slam at Speed +3 (5), you want to make sure of the hit.