I tried searching the Facebook archive, but couldn’t get the right text string without a ton of not the right results. 
In creating a 504 (or even IEP), if school says they see no impact (child is gifted and receives a good bit of enrichment so on level work isn’t challenging and those grades are just fine) and school also sees no evidence of the disability (because the kid masks and then explodes at home), is there anything specifically in section 504 that says a school must provide accommodations to minimize the triggering activities within the school building?

There are two requirements for a 504 Plan.  The child must have a disability and that disability must impact the ability to receive an education in the general education setting.  So you would need to track anytime the "triggering activities" affect the child's ability to learn.  Reach out to the general education teacher on a weekly basis and ask if there were any "incidents" with the student in the past week (explain what you see at home and that you're worried it might be happening at school).

Also, since you are specifically asking about how the triggering activities affect him at home, YOU need to track these incidents, why they occurred, and what impact they had on the child's education.  For instance, was he unable to do homework assignments?  This would be an impact on his/her ability to receive an education.

 

So the disability is Afterschool Restraint Collapse.  Get that documented and request 504 accommodations because it effects homework which is part of what needs to be done for school success.  I think it's going to be a fight.  (School should know that masking happens - especially with gifted kids.)