I have determined why installing the QHYCCD SDK enables my 294C to be detected and why the INDI packages do not. For reference, I'm using Kubuntu, variously 20.04 or 20.10, depending on the machine, but the same issues appear when using indilib packages only, namely:
lsusb does not properly identify the camera (it often shows up as 'Cypress WestBridge')
INDI fails to detect the camera at all
The package libqhy places a dependency on fxload, but the version available in Ubuntu and most other distributions is now a dozen years old. The SDK however has a much more recent fxload, which is included in its own folder named 'fx3load'. The SDK install script actually takes the rather forceful step of replacing the system fxload with its own fxload, while its uninstall script simply deletes it without reinstating the original (!).
What I did today was to uninstall the SDK, reinstall the indilib qhy packages and then manually copy across the SDK fxload to replace the system fxload... and it worked.
Until the various Linux distributions update their included versions of fxload that support the '-fx3' flag, realistically KStars/INDI is going to have to provide the functional binary.
That could be done by:
putting it somewhere else and changing 85-qhyccd.rules to point to it
giving it another name and changing 85-qhyccd.rules to point to it
Note that the Open Astro Project is another voluntary project; this is not an official QHY creation and it seems that his hope of some of his changes making their way back to the SDK are likely in vain.
Sadly, I haven't been able to use his scripts to create the tar file needed to create the .deb packages as the script hangs on a version name of a library.
At the user level, as a temporary workaround to get your QHYCCD USB3 camera working, you'll need to do what I did: