I see there was once in r45498 the ability to specify via "&uploadmsg=suffix" a custom uploadtext (which would be located at MediaWiki:uploadtext-suffix and subpages thereof). This was reverted in r46394.
While the change would not have "solved" the uselang hacks at the Commons (and elsewhere, en-WP used something similar last time I looked), it would still be useful with the new reupload form, because it would easily allow us to define a new uploadtext for reuploads.
Since the reupload form works differently (locked dest file name, different label and semantics for description, no license dropdown), it makes sense to have a different introductory text. It *could* be done through yet another uselang hack, but that'd be awful, since the uselang hack requires also to create subpages of a ton of other interface messages. It'd be far cleaner if we could have back this "&uploadmsg=..." feature. A sample uploadtext-reupload exists at [[:commons:MediaWiki:Uploadtext-reupload]]. (Currently not translated, since the feature doesn't exist anymore.)