Wikitext 2.0 proposed <ref#someid>we can do embedded refs <ref#otherid>this way</ref#otherid></ref#someid> which would allow optional hashes on extension tags which are completely ignored other than for matching. That would allow extension tag processing to "completely ignore the contents, just look for the matching close tag" as it does today, while letting extensions actually contain any content, not just "any content except the fixed-close-tag sequence". (If your content matches the close-tag sequence, pick another hash. You're guaranteed to be able to pick one that doesn't match anything in your content.)

The prefix-tag mechanism of heredocs (T114432) is a similar loophole for exactly the same purpose.