It wouldn't be ideal, but you could use a variation on light crumbs. Set up an invisible token with a 5ft aura visible only to players controlling that token - probably attach it to a character sheet so you can drag new ones onto the map as you need them. Give Bonkers permission for this character sheet. Now you can drag these tokens onto the token layer to mark the position of monsters (I'm assuming you still have the monster tokens on the GM layer at this stage). Only Bonkers and the GM should be able to see the coloured square marking the monsters position - GM can still see the token underneath. You can also set nameplates to controlling player only, so you could have the monster name there too. The limitations are that you can't have any artwork visible without other players seeing it (as far as I know) and Bonkers can also accidentally move the marker tokens. Not a huge issue as you have the real monster token on the GM layer so you know where the markers are meant to be.
