I do not know the rules of AD&D 1st edition that were in effect around 1987. I own, however, a copy of the Forgotten Realms Campaign Set (TSR1031). I’ve just noticed (after all these decades :D) that its DM’s Sourcebook of the Realms has alternative rules for subduing a dragon (on p.15): it can’t simply be subdued, it must be awake and challenged properly, and it must accept the challenge before subdual as per the general rules can happen. So… there must have been general rules for subduing a dragon.
What were the general AD&D 1st edition rules for subduing a dragon, and how would those rules best be ported to D&D 5e, if they’re worth porting at all?