Action 0x98 ShieldOn is used to interpolate the character into their shielding pose.
For Mario, the shielding hierarchy/animations have an extra part attached to his hand compared to the normal rig, not sure if this is the case for other characters.

#0 - neutral pose/hierarchy, position rig but don't animate

Misc 00 in the editor:
Import using "Import from Obj If Added" button.
Not sure if lighting enabled matters, probably not.
This is the pose that is transitioned into/out of for ShieldOn/ShieldOff.
It may need to differ slightly from the pose at the end of ShieldOn.
From my testing with Mario:
Part 00 reset translation/rotation to 0 when the shield pose starts, couldn't be changed by modifying the neutral shield pose(misc 00).
Part 01 retained translation/rotation from the end of ShieldOn, translation/rotation on the neutral shield pose is relative to the position at the end of ShieldOn
(for example if the final frame of ShieldOn sets x rotation of part 01 to -40, and the neutral pose also has part 01's rotation as -40, it will be -80 in-game.)
Part 03 retained translation/rotation from the end of ShieldOn, couldn't be changed by modifying the neutral shield pose.
Other parts were not tested, but at a glance it seems like the rest of the parts are positioned as expected in the shield rig, and only parts 00 and 01 need special care.

#1 - interpolate from right to top right
#2 - interpolate from top right to top
#3 - interpolate from top to top left
#4 - interpolate from top left to left
#5 - interpolate form left to down left
#6 - interpolate from down left to down
#7 - interpolate from down to down right
#8 - interpolate from down right to right

Misc 01 - 08 in the editor:
Length of each animation should be 45 frames.

The shield is attached to the last part in the hierarchy.
The position of the shield for each extreme pose(right, top right, top, etc.) is determined by points on an ellipse relative to the neutral position.
I've made an interactive graph which can be used to calculate the positions, found here:
https://www.desmos.com/calculator/hgtwj1sty1
Despite being points on an ellipse, the translation between positions is linear from frame 0 - 45.