I finally found the explanation of cracknel bread as saffron bread in, of all things, an old law book. The Law-French Dictionary Alphabetically Digested, to Which Is Added the Law-Latin Dictionary: Very Useful for All Young Students in the Common Laws of England (1701) has a rather random glossary, which includes the definition: Saffron Bread = Panis crocatus.
Of course! Saffron comes from the crocus flower, and crocus, or crocatus became cracknel! How wonderful!
A bit more information on both cracknel bread and simnel bread appears in The Gentleman's Magazine in 1866. The article is about the tradition in Bury, in the UK, of eating simnel bread on ‘Simnel Sunday’ (that is, the fourth Sunday in Lent.) It mentions both cracknel bread (in this case, it is a form of bisket-bread) and simnel bread, and includes a marvelously silly alternative story about the origin of the name of the latter.
The bread called "simnel bread" is mentioned by Jehoshaphat Aspin in his "Pictures of Manners, &c, of England" (now a very scarce work), page 126, quoting a statute of 51st of Hen. III.:—" A farthing symnel (a sort of small cake, twice baked, and also called a cracknel) should weigh two ounces less than the wastel (a kind of cake made with honey, or with meal and oil)."
Alderman Wilkinson, of Burnley, a well known able Lancashire antiquary, some time since stated that it "originally meant the very finest bread. Pain demain is another term for it, on account of its having been used as Sunday bread" (if a conjecture may be hazarded, it is possible there may be some connection with the shew bread and heathen votive offerings, as in India and China) "at the Sacrament. The name appears in mediaeval Latin as simanellus, and may thus have been derived from the Latin simila = fine flour. In Wright's ' Vocabularies' it appears thus :—'Hic artӕocopus = symnelle.' This form was in use during the 15th century. In the 'Dictionarius' of John de Garlande, compiled at Paris in the 13th century, it appears thus :- simeneus = placentӕ = simnels. Such cakes were stamped with the figure of Christ, or of the Virgin.
It is not a little singular that this custom of making these cakes, and also the practice of assembling in one place to eat them, should be confined to Bury. Such is the fact. No other town or district in the United Kingdom is known to keep up such a custom. As stated above, much labour has been expended to trace its origin, but without success. Some years ago a sort of Eclectic Society in that town, who used to hold meetings on Sunday Evenings, gave notice that they would discuss this question on the coming "Mid Lent Sunday Evening." They met in an old room just out of one of the principal streets, and the chair was taken by a master-hatter, who afterwards became a Baptist preacher. Much laughter was caused by his explanation respecting the origin of the term "simnel," which he said, he had heard, arose from this circumstance: "In an old part of the town called ' the Island' (a plot of land nearly isolated from the Irwell), there formerly resided an old couple, who kept a small 'toffy-shop,' which was famous amongst the schoolboys, &c, for a peculiar, and, to them, excellent kind of sweet cake. The names of this old couple were Simeon and Ellen; but, according to common Lancashire parlance, they were usually addressed as Sim and Nell, and thus the cake came to be called ' Sim and Nell's' cake—easily corrupted to 'Simnel cake’! This, however, did not explain the practice of eating the cake during Mid-Lent only. It may be added, that the Monday following is often accounted a holiday, and that the word "simnel" is vulgarly pronounced ‘simblin.’
I hardly have any choice but to offer you, as the recipe for the day the third (or is it the fourth?) sort of cracknel bread, the version richly studded with pork cracklins, from the American South, do I?
This is the portion of the fat meat which is left after the lard is cooked, and is used by many as an appetizing food. The cracknels may be pressed and thus much more lard secured. This latter, however, should be used before the best lard put away in tubs. After being pressed the cracknels are worked into a dough with corn meal and together made into cracknel bread.
Home Pork Making (New York and Chicago), by A. W. Fulton