shift-key "passthru"

I've set up a layer that gives me access to certain accented characters şçêûî using compose-key-sequences.

Now I'd like that if I press Shift, it will give me the capital versions of those letters. One way to do this is to overwrite the shift key on that layer with another layer toggle for a layer that gives me access to the capital versions.

But this would only work if I first press the original layer-toggle and then press shift. If I press shift first, it won't have the right effect.

Is there a way to "pass through" the status of the shift key to the key at the end of the compose sequence?