A loaded question. There are a few variables at work, including your level of analysis. The short answer is that using ʘ, ǀ, ǁ, ǃ, and ǂ is a well-established practice for Khoesan languages. The letters c, q, and x are used for clicks in the Nguni language family, which includes Xhosa and Zulu.
Click phonemes are used in Khoe and San languages, traces of some of the deepest roots of the human experience of sharing this planet. Anthony Traill (1939-2007), a South African linguist at Witswatersrand, analyzed up to seventy-nine clicks and click clusters across the twenty-seven languages in the Khoesan family. For linguists, these are represented using combinations of letters, including several from the International Phonetic Alphabet: ʘ (bilabial), ǀ (dental), ǁ (lateral), ǃ (alveolar), and ǂ (palatal).
Do these characters ever appear in print? Yes! They have long been a part of the standard orthography of most, if not all, of the Khoesan languages. Since 1979, their use has also been standardized as part of ISO 6438, the Coded Character Set for African Languages in Bibliographic Information Interchange, now superseded with the equivalent characters supported in UNIMARC and Unicode.
This is not to say that the standard has always been met with adequate system support on implementation. As the level of support has varied, workarounds have been tried–sometimes by writers, substituting | and ||, or / and //, for ǀ and ǁ, or ! for ǃ (yes, those are different). One widely used piece of library software (which will not be named here) uses the palatal click as its subfield delimiter. Not a mere bug, this: it’s a tarantula.
Three clicks also occur in the Nguni languages (e.g. Xhosa, Zulu, Swazi), where the orthography is simplifed to representing them as ‘c’ (dental), ‘q’ (alveolar) and ‘x’ (lateral).
With the wider availability of Unicode, the potential introduction of RDA, increased attention to localization of software and the advent of cloud computing, it looks as though the bibliographic situation for languages using click characters will soon become much clearer. For legacy systems that do not support click characters directly, there are proposed workarounds that could be used, borrowing from the orthographic model as used for Nguni languages. But as yet, there’s been no provision for the bilabial click proposed for use in following this option.