Erik Darling shows how READPAST is no panacea:
Locking hints can be really handy in these situations, especially the READPAST hint. The documentation for it says that it allows you to skip over row level locks (that means you can’t skip over page or object level locks).
What it leaves out is that your READPAST query may also need to try to take row level shared locks.
Read on for an example as well as an alternative which ends up being better in this case.