Looking for help/suggestions how to implement a random dungeon deck

I am looking at how possible it would be to implement Warhammer Quest user Roll20. I'm confident most elements will be possible to translate, except for the dungeon deck sue to it semi stacked setup rules. Quoting from the manual. "Shuffle the dungeon cards and deal out six face down. Now take your chosen objective room card and shuffle it in with the six cards you have just dealt. Finally, deal a further six Dungeon cards on top of the seven you already have. You should now have thirteen cards, and your objective lies somewhere among the final seven... but who knows where exactly! The remaining Dungeon cards are NOT used, and may be returned to the box. The thirteen cards, including the objective room, form the Dungeon deck for the adventure." Any suggestions how to achieve this would be appreciated.

make a second deck that only has the objective rooms in it, then keep track of how many cards you have drawn, once you hit the point at which it should be the objective (it it was a 25 card deck, it would be the 22nd card for example) then just draw the dungeon card. Instead of making these a deck of their own, they could just as easily be a "token" which can be placed on the table.