Suggestion Box

But I realise that can be misleading, in lists it'll show up as a contest, right?

Yes, and it's a pet peeve of many to look through the Contest genre and find contest entries instead. In the event your suggestion isn't implemented, here are some ways you could go about identifying pieces written for a specific contest or contests in general:

Create a folder in your portfolio to hold your contest entries.
Add a note at the top of your item body, something like "Written for {item:#}," where you can replace the pound sign with the contest's ID number.
Mention your piece is a contest entry in its brief description. You'll need to write out the name of the contest, though, because WritingML won't work there.