The Evasion Tom is staging is modeled on the imprisonment and escape of noble prisoners in romances like The Count of Monte Cristo, so he insists on making Jim a coat of arms. The "chain embattled" is included, he says, to represent "slavery." Note how the "crest" imitates the stereotypical image on a fugitive slave wanted poster like the one the Duke prints up.
When Jim is disguised as a "sick Arab" in Chapter 24, the Duke dresses him up in "King Lear's outfit." Kingly clothes and a coat of arms -- these sound as if they could be meant to evoke the idea of Jim's innate human nobility, but in Kemble's drawings both make his figure even more clownish.
The Barrett Collection, UVA PS1305 .A1 1885b