Serving Simplified Chinese to a user who has requested only Traditional Chinese, or vice versa, is a cultural faux pas. The two character repertoires, though mostly mutually intelligible with some mental effort or explicit learning, aren't similar enough to be freely interchangeable.
A typical Hong Kong user, for example, is more likely to be comfortable with English as a fallback than with Simplified Chinese. English is taught in Hong Kong schools. Mandarin instruction, though common, would not necessarily be done using simplified characters. Hong Kong users who prefer Simplified Chinese as a fallback would likely have configured their browsers with zh-HK, followed by zh-CN, followed by en. That said, serving Traditional Chinese text that was written for a Taiwanese audience would be quite acceptable.
Specifically, if you have Simplified Chinese and English versions of a text available, and a request comes in with
Accept-Language: zh-HK, en, the only reasonable action is to serve the English version. If a request comes in with just
Accept-Language: zh-HK, then I would consider both versions to be equally foreign; whether serving the Simplified Chinese version or the English version is better is a matter of opinion.
Likewise, for a client whose only configured language preference is zh-TW, I suggest choosing English as a fallback over Simplified Chinese. Again, a user who is comfortable with reading Simplified Chinese would likely have configured zh-CN as a fallback. For you to choose Simplified Chinese even when not requested might be perceived as a political statement. Whether or not it is acceptable to serve zh-HK content to a Taiwanese reader depends on the register in which the text is written. In general, it should be fine. However, if the text is more of a transcription of colloquial Cantonese rather than in the standard written register, then a Taiwanese reader would likely not be able to make any sense of it.
The same guideline should hold for Mainland Chinese audiences as well. General education is done entirely in Simplified Chinese, so I wouldn't expect any familiarity with reading Traditional Chinese.