I'm not sure if you have an Agway feed/aggricultural supply shop by you, but I have been impressed with a sand mix which is sold in bags there. It is called Multipurpose Sand by Yard Right Select. I keep it moist, which facilitates burrowing, and my monitors readily excavate burrows in it.
One thing that you might want to consider, is utilizing an acrylic/plexiglass covering for your cage. If you are using a screen type top, moisture is escaping, which is drying out the soil. In all honesty, if the moisture/humidity's right,they can dig a burrow in almost anything. If the substrate is drying out, these burrows do not hold for too long.
As for the compost, what type of compost are we talking about? Leaf litter, animal waste, kitchen debris-eggs shells, banana peels, etc..? I don't see a problem with using composted leaf litter, although you might want to cook it in an oven to kill off any mold, fungus, or microorganisms which are probably residing in the rich medium. I would stay away from composts that have decaying animal waste-dog poo, and decaying food, etc.. I think that that sort of mixture would just invite bacteria to flourish and overrun the enclosure... just my opinion.. hope this helps a little, take care,
here's a pic of the sandy mix I have been using...if the pic doesn't work, it is also under Kimberly Caging on my website.
bob

The Odatriad