Fetching SHA id's instead of named references?

Hello

Is there a way to fetch based on SHA id's instead of named references?

My usage scenario is this: A change management tool based on version controlled manifest files (somewhat similar to Google's Repo) must be able to check out exact versions of all 200 trees in the project view. To support this, tags are used since they specify an exact revision. But there are two problems with tagging:

 * Tags are not immutable.
 * External components that already have a tagging style get polluted
by our excessive use of tags.

I would really prefer to just list SHA keys in the manifests, but fetch apparently doesn't support that? Could I use a combination of lower level commands instead?

BR / Klas