Feature request: Specify hooks on the command line

I'm developing a project called [Plushu][], an ecosystem for creating 
plugin-based dedicated user shells, patterned after [Dokku][] (and 
similar to git-shell).

[Plushu]: https://github.com/plushu/plushu
[Dokku]: https://github.com/progrium/dokku

Plushu has its own hook system for plugins (so multiple plugins can 
listen on a single hook). Right now, to extend Git's hooks to Plushu's 
plugins, Plushu's plugin for Git links the "$GIT_DIR/hooks" directory 
for all new repositories to a directory with scripts that delegate to 
Plushu's hook system.

I'd rather not have to alter the repository directories to create this 
interface, so it would be a significant improvement if Git provided some 
mechanism to specify hooks on the command line. I was thinking something 
like a GIT_HOOKS_DIR environment variable, or perhaps a "--hooks 
update=/opt/hook"-style option.

(Perhaps, as a more general solution, there should be a way of 
specifying alternatives for individual GIT_DIR subdirectories per-command?)

I'm willing to draft the patch for this if someone could point me to the 
relevant spot(s) in the code.