I can't imagine the wagon roof adds significant structure (I mean, that would cost AMG a lot to differentiate the models so much), it seems like the Opentop rollbars are just boxed out in the wagon.
EDIT: I've come up with the following documents that may or may not pertain to the opentop's roll-over protection. These documents may refer to the Uparmored models... I've emailed AMG. I'll let you know what they say...
This document is of particular concern to me:
This is from 1999. Have these updates been made?
"Rollovers were addressed in this analysis but are so dangerous and deadly that they also
require special emphasis in this report."
HMMWV Seat and Safety Evaluation
Also, this is a concern:
This is from 08/2005!
Declassified Army Musings
"[suggested] HMMWV safety upgrades – five point seatbelt, roll-over protection, gunner restraint, VIC-3 intercom system, emergency rescue/egress & body armor "
However, this is reassuring:
HMMWV Handbook
"14. Extreme caution shall be taken when transporting personnel. Rollover protection and seatbelts are available for the crew area only and is not provided in the troop/cargo area. (Failure to use basic safe driving skills may result in injury or death to personnel and damage to equipment)."
Here's another, but this one may clarify things (or not):
"Determine quantities needed & produce rollover protection kit for soft top HMMWV models (M998 / M1027) (existing design; Lead PM-LTV)"
This applies to:
I think. Though the family is M998 for HMMWV's.
Another Study