Is the usage of a hyphen in "ad hoc" acceptable?

The rule-of-thumb I've found in researching this issue, (though no reference to a specific style guide was referenced – one site linked here) is if Latin and other foreign phrases are not hyphenated in their original language, then they are not hyphenated in English.

"ad-hoc" is wrong; "ad hoc" is correct. The book Grammatically Correct (2nd edition) by Anne Stilman mentions this as a commonly mis-hyphenated compound, along with the following (correct usages):

• more or less
• ongoing
• under way
• a priori
• bona fide
• post hoc
• vice versa

The same book provides further guidance on hyphenation, a good reference.