Like so many other props in game, could always provide the stocks and cage, and allow the garda players to use their discretion, deciding when and how to use them. There are a number of veteran players in the garda faction capable of good judgment.
Someone could argue that locking criminals up in the citadel would only invite high level characters to go crazy in attempts to set criminals free, which has happened. However, just because some players have gone overboard on occasion with the citadel, it doesn't follow that the citadel shouldn't be used.
While the concerns of potential ridiculousness have merit, and just like other things, ridiculous incidents would at times occur, there are also solid positives with providing stocks and public cages.
I'm reminded of a great scene once, in which a garda had a criminal, and knowing there were a group of "caliban" on the outskirts of the outskirts, the garda approached and flung the naked outlander towards the group of "caliban." Feeding the scum to the wolves, so to speak. Similarly, the garda could sentence one to a night in the cage or stocks, and if they survive until the morning, they must have good friends and luck both.