Is there a way of creating a BETWEEN feature for Powerlists? I.E. I want BETWEEN X and Y days.

This should be possible by using two date filters, one with greater than your oldest date and the one with less or equal to your soonest date. If you want a rolling window, you could do the same thing but incorporate the "Subtract X days from Rule Date" feature.

Happy to help if you have more specifics!