This is NOT about making this available on a wide range of devices/apps. The OP very specifically stated that it was for their own personal use on their own device which supports this functionality...
All Kobos are quite happy to render font-variant:small-caps; as long as your book is a kepub (i.e. everything Kobo sells). The small-caps will be 'simulated' from the font currently in use rather than a true small-caps font.
If the OP wanted to read the book as an ePub on his Kobo, then your technique of forcing lowercase on the entire phrase before applying small-caps still wouldn't work...according to you, they would "have to change the case back to
uppercase in order to have it work with simulated smallcaps."
Your statements seem to argue against your own previous statements...you keep saying that small-caps ONLY works on lowercase...now you are saying they would all need to be uppercase???
Anyway...I am also officially
DUN with this particular argument. Cheers!