Can “未” modify descriptive adjectives?
Yes, definitely.

Disclaimer
I am not nearly as proficient in classical Chinese, so my answer is limited to modern Chinese. 

IMHO, 未 is more commonly seen in formal context and is often used with 曾 or 從, as in 未曾 or 從未.

For example,


  單純的你，未曾憂鬱過。


You're such a simple person; you've never been depressed.


  人生自古以來，從未順利。


Since the ancient times, life (of a human) has never been smooth.

Note that the adjectives act similarly to a verb, as suggested by WangYudong above. I don't think 長 and 短 are verbs per se.

EDIT:
FYI, as FortCpp explained below, adjectives in such construct may actually be verbs.