A dressing gown belt or a tie works just fine in place of a Yoga Strap.
Freeing places we tend to get stuck or locked helps us to find more movement and engage more muscles - in this case the abs. Often when we stabilise in one place we find movement in a place that may have been a bit stuck. See if this concept assists you on the road to the Pilates Rollover.
Also I had a question about hyperextension of the elbows in weight bearing - there's a great tip in here to help find stability here.
Enjoy!