Variables can indeed bleed into sub-processes if you're not careful. If the sub-process's command is declared as a dependency then 0install could reset the environment automatically when starting it. Currently, it instead finds a set of libraries that work with the process and its dependencies (which is safe, but could make some cases unsolvable in theory, if a program wanted to run a subcommand which used an incompatible library).
Direct support from the dynamic linker would be nice though.
Variables cannot be set in the child process; the linker will have already run by the time you can access your environment. Furthermore, this scheme destroys the utility of LD_* variables, which itself is a non-starter.
All setuid binaries will be incompatible with any scheme which involves environment variables controlling the linker -- that's a core element of unix security architecture.
Over here [ http://news.ycombinator.com/item?id=3995200 ] someone who I believe is the author suggested that this thing uses DT_RPATH, and I've outlined the problems with that mechanism as well.
Perhaps I'm coming across as a curmudgeon, if so I apologize. But I've worked on this problem quite deeply and there's just no way to reliably solve it. Modifications to ld.so are an absolute requirement.