The seizures are involuntary. Similar to what happens during partial hanging. Your body may disconnect movement during sleep, in most cases unless you have sleep issues, so you don't react to dreams, but when your brain is dying due to oxygen deprivation the seizures are involuntary and generally (or almost always) occur.
This report of the inert gas method (hypoxia), undertook over a rather generous sample basis (121 cases), suggests that seizures (confirmed after unconsciousness achieved - the 2min mark being the max) happened only 50% of the time, which would be more optimistic.
Their observation didn't describe this stage as brutally violent of an earthquake nature but manageable, not extending too long in time. The leg and arm movements seemed to indicate a lack of amplitude and some weakness.
This is hopeful. Of course, it will be wiser to prepare for the nastiest scenario (epileptic Ian Curtis dancing at its prime)
Brainstorming out loud, with some out of sight shameful simulation exercice
Positioned lying flat on one's back, since the ReBreather permits it, the advantage would be that it's impossible to fall below ground level (except to Hell), arms and legs wouldn't be safe from surface collision but not impactful. The head may be more exposed than not leaning against something. The straps should be well fixed and resist to friction. For the face mask to hit the ground from body movement, it would require a cumulative torso plus neck torsion at the same time, cause when the neck is relaxed, it hardly turns past 75 degrees left or right by itself. Unlikely.
Compared to being seated, lying on the back, the arms may meet less resistance, be more free of movement due to physics and context (rotational drive for given amount of force), but at a out of reach distance for the hands to involuntarily hit the mask in place. More likely, the tube area would be closer to reach, with the carrying bag interfering somewhat along the way. SM enthusiasts may consider handcuffs (joking)
Being seated, if the animal is quite short / voluptuously breasted ? and the carrying bag, attached to torso, bulky, may be impractical (to verify). Still being seated, the hands may become at an adequate distance to reach the middle of the head, depending on head to torso angle, but as reviewed before, unlikely that Mike Tyson uppercut punch trials beat the gravity.
Finally, in the video instructions, the length of the tubes seem short, hope they come with enough freedom excess to satisfy any funky position / movement, without causing displacement or dislocation (to verify again)