As indicated above, since the army S3s met all the ADRs, then certainly by duplicating their seat belt installation, any S3 could be operated with the roof off except, as noted above, the five door wagon, and should not need engineering. The same could be applied to any two door 110 by duplicating the army roll cage up to the last time the army acquired 110s that were the same as the civilian model (1989?). For later 110s or 130s (or even 90) it would require engineering approval for the change, and the key problem would be the seat belt mount, but something similar to the army setup should be easily approved.
Removing the roof on any five door wagon would require engineering approval for the changes to the B pillar, and this would need major structural reinforcement.
Series 2/3 doors with removable tops fit any 90/110/Defender, but as mentioned in an earlier post, no rear or back doors on any five door wagon have removable tops. The back door can be replaced by a drop S2/3 tailgate on any Defender with the standard tub.
John
