Indeed, in this specific case dropping the pas is the normal formulation (it used to be that the negation was expressed solely via ne, since the schwa is now rarely pronounced and ne does not carry accent, pas nowadays tend to carry the weight of the negation). It's not impossible that the word may be restored to the expression in the future, but it is not a current usage at all.
It is frequent for specific expressions (in all languages) to preserve otherwise obsolete fragments of syntax, grammar and meaning (English example include fight with, where with originally meant "against" and hold fast where fast means "firm, secure" and is an adverb without -ly). In this case, the specific construction falls within a broad group of verb+noun expressions where the noun isn't accompanied by an article (Grevisse, Le Bon usage 14e ed., §587 c).
It does present an interesting peculiarity among those expressions: the adjective. Most other cases are constituted of a simple verb+noun pair, and when a qualifying adjective is added, the article reappears, which is not the case of the expression ne pouvoir rêver meilleur X. As a personal hypothesis, the expression might be an extension of an earlier ne pouvoir rêver mieux, i.e. the noun phrase reflects what was originally an adverb.
Many verbs have or still show hesitation as to the proper way to construct them (just like some do in in English). In fact rêver still shows hesitation as to whether the correct preposition is à or de! It was once more common for that verb to also be constructed as a direct transitive verb, especially in the meaning of "meditate, think about, imagine, desire", which this expression may well have been derived from (Grevisse, §296 F). Today the usage is at best highly literary anywhere else but in this expression.