superlative + -ing participle + noun ok?

Is it always ok to have a superlative hyphenated with a present participle ending in -ing acting as an adjective (so long as the superlative describes the base verb of the participle)? For example:

• most-burping woman
• fastest-drinking college student
• quietest-singing bird

Such a phrase that is commonly used is "highest-grossing film." Is this just a particular idiom?

It is, but the hyphenated version will often give a meaning different from the unhyphenated one. The quietest-singing bird is the bird whose song is the quietest of all birds. The quietest singing bird is the singing bird who sings most quietly. Without the hyphen, highest grossing film would be absurd.

The Wikipedia entry on English compounds notes:

Comparatives and superlatives in compound adjectives take hyphens:

• "the highest-placed competitor"
• "a shorter-term loan"

However, a construction with most is not hyphenated:

• "the most respected member"

Compound adjectives that include comparatives and superlatives with more, most, less or least are normally not hyphenated.

So, by these guidelines:

• The most burping woman
• The fastest-drinking college student
• The quietest-singing bird

Highest-grossing film isn't an idiom, but simply a superlative compounded with a present participle being used as an adjective, and is therefore hyphenated.