This is a catch-all to focus the common theme of:

• It is not possible to set Starlark defined command line flags that come from a different repository #9065 (fixed)
• Custom build flags choke on prefix workspaces #9177 (fixed)
• Using new string_flag settings in external repositories #10152 (fixed)
• Broken repository name handling for build_settings and label_flag #10499 (fixed)
• Toolchain resolution stumbles over workspace prefixes in constraint labels #10927 (@katre is looking into this)
• Add support for user-defined build settings in external repositories #10975 (fixed)

At the narrowest this may require unrelated fixes to build_setting and toolchain resolution logic. But the general theme of the problem is clear, however the fixes are made.

We're (Bazel configurability) trying to prioritize this for the current quarter.