Any experiences are welcome.
I took a couple runs at downshifting, but not at a Mega Corp. I tried to frame my discussions around how the changes would support business priorities. In my case that would be having me available at a lower base cost with the option of coming back to FT as needed. For a smaller company that is attractive. If you can find ways to show that downshifting is beneficial for your part of the Mega Corp do that. If not I'd try and find some generally acceptable personal reasons for doing so I'll list some below that come to mind:
1. Working on personal educational/accreditation/development goals.
2. Personal health issues.
3. Supporting family members.
4. Working on NGO/charitable projects.
If you are close to FIRE and have lots of FU money so losing your job is not a huge issue you can frame the issue more aggressively and offer to work PT or leave. Obviously the more important to the organization you are the easier it is to do this successfully.
Does your organization have any sort of sabbatical type program? If so look into that as an alternative to downshifting should the downshift request not get traction.