While the column "BaseAC" now is functional for Small, Large and Tower shields (1, 2 and 3 AC respectively), you cannot add more shield lines. They have several hardcoded game aspects such as their animations, position where they appear on the forearm, their interaction with other weapons.
In addition Tower shields are hardcoded to be unusable by smaller creature sizes than Medium.
Animations - as per below - and the fact you can drag potions (line 49) to a henchman to have them drink it is hardcoded. They also disappear on the last use even if they have charges or uses/day added, and the game assumes only one usable item property when dragging to the quickbar.
Flails, Morningstars and Whips
Fails (light and heavy) and morningstars cannot be equipped in the off hand due to engine coding. They also never use the polearm "over the should at ease" rest animation, which Halberds and other polearms do, likely due to animations or clipping.
Flails (light and heavy), morningstars and whips have different positions of their weapon VFX hardcoded, see: iprp_visualfx.2da for some details.
Hardcoded Use Item Animations
There are some "use item" animations that are hardcoded: