I thought 아/어라 normally meant an imperative, but in "아, 예뻐라!" that wouldn't make sense. Is it more like an expression of surprise or admiration? Is it a general pattern that can be used with other descriptive verbs?
Yes, when "-아라/-어라" attaches to descriptive verbs (a.k.a. adjectives / 형용사), they make the sentence an exclamation (감탄문), in the same way as "-구나". Examples include "어머 예뻐라", "아이 추워라", "참 착하기도 해라", etc.
- Note that in the last example, "하다" is an auxiliary adjective (보조 형용사), not an auxiliary verb (보조 동사). Contrast with "빨리 오기나 해라 (Just come fast!)" where it is 보조 동사.
In fact, in prescriptive grammar (i.e., school grammar 학교 문법), this is the only accepted meaning of "-아라/-어라" combined with adjectives, because adjectives are not allowed to take a command form.
In other words, commonly used expressions like "행복해라" (Be happy) or "건강하세요" (Stay healthy) are considered wrong by prescriptivist grammarians. See, for example, these posts by 국립국어원: 평생불행해라 / 건강하세요, 행복하세요
- If you ask my opinion, I think expressions like "행복해라" or "건강하세요" are perfectly fine, and I think the majority of Koreans will have the same judgement.