I have done the same thing (for a different reason, obviously) as I offer to share my SSL with my website clients. Since they, too, are in cPanel Addon domains, their clients can access via https://MyDomain/AddonDirectory/SecurePage(s). That does not generate a warning although it does display my domain as the SSL's owner but it does allow the clients' secure page(s) to be secure (under my SSL). It's worked fine for many years.
What I perceive that you've done is simply force an https:// protocol on client pages which, while providing an encrypted link (I hope it does that), browsers dutifully report a non-secure (not validated by an SA) situation. It's as if you've "self-signed" a certificate for your clients.
IMHO, if they need an SSL, charge them for it and do it properly.