One of those tough management situations. No "one size fits all" answer will work, but perhaps the following principals might be helpful.

I noticed you want to "implement standard practices" but because of what I understand to be an matrix organizational relationship you can't really direct.

It has been my experience in these situations that instead of coming up with a great plan and then figuring out how you get these local HR representatives to implement your plan that you should let them come up with the plan.

Specifically, bring them into the process at the beginning of the discussion. Don't walk in with any perceived notion of how things should be...rather identify a problem that exists across your plant(s) and bring the HR team together to work through possible solutions. You become the moderator for change as opposed to the implementer of change and you are left with HR staff who now own their own changes (of course depending on a long list of variables I might not be aware of).

Give that a shot and see how it works.