In short, if we introduce the downstream versioning, we will ship
library with ABI incompatible with upstream ABI. If we won't,
applications with modules/plugins (ie. Apache with mod_perl) that each
use different version of the library may break due to double symbols
(one from the v5 and other from v6, and ld would not know which symbol
is which).

I'd like to ask for suggestions on how to resolve this problem.

As I stated above, I welcome any suggestions. I would also like to hear
from someone responsible for the distro architecture which of the
aforementioned issues is more painful for us, so we know which path to
take if no complete solution is found.