I left a warning on the post page, though, since I don't think it was otherwise obvious enough what was going on.
When I click on Quote in a locked thread, I see this warning.
This topic is locked, so you can't actually reply!
With your worry on this warning is not obvious enough, that I agree with you, why don't consider to add color for this warning text?
Some colors for consideration like red as it is used for other warnings, when making posts in inactive thread last 120 days; or other colors like brown or purple like you would prefer to use in your post when you highlighted something.