If a first post or late answer is deemed bad, I feel flagging it as low quality is the correct step. But the post then ends up in the low quality posts queue, and only there the deletion-comment can be selection. Wouldn't it be better if one could select this comment already when flagging for low quality in order to simplify the reviewing process for the low-quality reviewers?
It would complicate Stack's software. You need far less reputation to review first posts and late answers than you need to cast low quality close votes. Making it possible in one place for higher reputation users would be nice, but is it really worth the effort? I like your idea, I just don't see it.