Chemical "injection" is used in many applications, but only a few with the minimal requirements I associate with the carwash industry.  Many of the so called "injection" applications require not only tightly controlled additions, such as acid to control the pH of pulp in the paper industry, but also rapid mixing associated with applications involving chemical reactions.  Many situations where liquids are combined in a pipeline are followed by an inline or static mixer to accomplish more complete mixing than associated with "injection."  Some liquid additions actually involve high pressure impingement mixing to accomplish sufficiently rapid mixing.
Without some specific features that take chemical injection to some level of specialization, simple addition through a tee or other injection fitting is sufficient for the simple applications.  I don't think that the world is desperately looking for another low pressure, moderate control, type of chemical injection.  Most of the easy applications are handled by conventional means.