I hope something can be worked out.
Debian stable releases (and their users) require long-term security support for whatever version is packaged, and that’s a deliberate choice for their use-case. It shouldn’t be that hard if the timing is well co-ordinated, such that the latest feature-complete version lands in Debian shortly before freeze (November 2016), and upstream made some long-term (2-3 years?) commitment to identify (e.g. file CVEs) and try to fix security bugs in that version.
The only ways to get that wrong, are if a rapidly changing development version went into Debian sid/testing too soon (and thus requires substantial changes after the freeze date); or if upstream doesn’t clearly identify or make easy to backport patches for security bugs. (e.g. Oracle MySQL, not detailing the security bugs fixed, adding arbitrary new features in a maintenance release, and not even having public source version control ).
By all means call the long-term supported version by some other name / numbering scheme if you want to differentiate it from the very latest vendor release, but please don’t use trademark law to threaten distributors.
Also, Debian users expect a smooth upgrade between major versions of the OS, so that needs to be supported by applications too.
The former owncloud maintainers can be contacted at pkg-owncloud-maintainers@lists.alioth.debian.org