Just a guess, mind, but perhaps someone (nuncle?) would have the resources to check it out.

Well, the i in both wilde and wildernes is, in Old English, short, according to my Anglo-Saxon lexicon. The marking of a long vowel with a final -e (whether not pronoucned or, as in Middle English, reduced to a schwa) is a Middle English or Early Modern English convention. If length was marked at all in Old English it was with something that looked a lot like an acute accent. Vowel length can also be determined / reconstructed by a words use in poetry (prosody, metrics) or comparison with cognate words in related languages. Because the original i was short, the vowel shift doesn't really explain why this stressed syllable became a diphthong in Present-Day English. It could be from analogy or just a plain fluke, as others have suggested. The OED1 seems to suggest that the wilder in wilderness may be the comparative form of wild, but it may also be from an earlier compound wild-deornes 'wild animal (i.e., deer / German Tier) ness'. (The gloss on wildernes was 'brutish, bestial'.)

Ceci n'est pas un seing.