Since this is an aspect of system programming, the answer is a little bit long as there are several parts to it.
I have looked this up and I am certain that I understand the position. I have reviewed the detailed format definitions for both WAVE and RIFF (including the provided C++ sample code and header files).
Identification of metadata blocks is contained within the generic definition of RIFF, but not in the specific limited list of FOURCC identifiers for the WAVE subset.
Most (if not all) WAVE file renderers (including the Microsoft ones) are actually RIFF parsers, but ones that only interpret the WAVE subset data blocks (they ignore other RIFF data blocks with FOURCC identifiers that are not in the WAVE subset).
So using the underlying RIFF
As Big Bill says, in practice, Ogg Vorbis comments can be included.
In practice, so can ID3 (two different types).
In practice, so can any other Metadata anyone cares to define within an RIFF metadata block.
But these are relying on the underlying RIFF structure: as far as the WAVE format is concerned they are undefined & unknown RIFF blocks, as the FOURCC identifiers for them aren't specifically defined within the WAVE specification.
In other words within RIFF datasets you can choose to use whatever metadata format you want, provided it's contained in an appropriate block. However, whether any specific reader can understand the format you choose is an entirely different question. It doesn't have to read any metadata blocks.
The WAVE format doesn't help with this as it doesn't even define a FOURCC code for metadata (that's only defined at the RIFF outer layer and WAVE doesn't include the entire RIFF set of data block formats).
There is no practical reason why you should not include metadata within WAVE files; it simply may or may not work.
There is no onus on anyone writing a WAVE reader to read any metadata block at all. There isn't even a definition of what format to use, should they even try to read it. If it fails to read it or reads it wrongly, that's your hard luck, it's not a fault in the reader (unless the manufacturer specifically says it will read a particular form of RIFF metadata).