Dear Myrna,
I have tried to make some research on the possibility that Bosch used secret codes in his artwork in order to convey some concealed message to its viewers, but if there is something in that direction, it has long ago become lost; at least, I have not been able to find anything of that sort in the little spare time that I have at my disposal, save for a few suggestions from certain scholarly quarters that he was secretly trying to undermine the Christian faith through his art, which of course strengthens such view.
That he used codes now seems to me perfectly possible and of course a fascinating subject, since the esoteric character of Bosch work is to me, if I can use the term, almost visible. What is certainly true is he, like some of his contemporaries and followers, did make a prodigal use of proverbs that were common at his time to express the way he perceived the people's general behavior and many of their peculiar (and very frequently not very saintly) ways. A lot has been said about this particularity both in this thread and elsewhere, even though those proverbs and puns are no longer known at present. And if he at all at any time resorted to secret codes to achieve some hidden purpose, I am afraid the keys to understand their meaning would at any rate be no longer known.
Of course, some of the proverbs have been identified. Some of them are very common and simple, like that of the big fish eating the small fish in the central panel, and others. The dogs on the right-wing panel are a fascinating enigma to me too; they could represent major vices or sins, like the seven cardinal sins making prey of man. I believe the word "slaves" in your post is an important clue to the possible meaning of the characters in this section. For instance, an hourglass could mean man has become a slave of time, and other devices might represent slavery of any stupid hobby or activity. This perfectly becomes to the men of our time. The hellgates are always open and welcoming hundreds of sinners to their dreadful destination. However, there are so many possibilities that we would need a lot of that precious time to just begin to disclose them.
As I have said so many times in this thread, I believe the message that Bosch was trying to forecast to the world, and perhaps to viewers in the Earth's "last days," was the same old admonition by all the old great biblical prophets: "Make your ways right, forsake your false gods and idols, lest you and the land shall perish."
Thank you very much for this fascinating contribution of yours, it has made me think a lot.
Best Wishes,
Luis Miguel Goitizolo
PS. Here is a video I found while I was researching for this post. I hope you and all will like it (the music is so great too)