Cascade will do fine. It won't be the same, but it's a lovely hop to dry hop, and at pretty much any other stage too.
Cascade as a dry hop will give you light pine and fresh citrus aromas. The Ella on the other hand has more of a fruit/stone fruit aroma, no citrus or pine IIRC... but not particularly pronounced. Goes better as late additions. Although it has been a while since I used it...
I reckon you'll end up with a nice profile. Not a smash anymore, but probably a better beer for it!
Cascade and Ella seem to work really well. Ella is quite floral as well as a little stone fruity.. anyway cascade seems to compliment other bold hops well I reckon