Hi Sops O' Wine,

I don't know of anyone who might be an experienced expert on this matter, so I'll offer my personal 2-cents' worth.  On our property here in western Lewis Co., Washington, we have a fairly typical assortment of the native trees and shrubs.  Woods that I've chipped either for composting or for mulching have -- not uncommonly -- comprised Cascara along with Doug Fir, Red Alder, Black Cottonwood, willows, and to a lesser degree branches from Oregon Ash, Western Red Cedar, Big-Leaf Maple, Vine Maple, and so forth.  I've never noticed, nor would I expect, any kind of negative effect in the use of Cascara chips.  If you have any specific concerns, you might want to set them aside in a separate pile to weather for a year before using them or mixing them with other mulching materials.  

When it comes to toxicity concerns of mulches, the only plants that I can recall hearing or reading about are Oleander (--certainly not a concern here where we can't grow them --) and Walnut, due to its purported allelopathic effects to plants growing around it.  I presume that there may actually be some research behind those claims and that they aren't merely horticultural voodoo lore.  What I can say is that I've never noticed any problems in the use of Cascara wood chips and I wouldn't expect any.  

When it comes to chipping and using woods here on our property, however, I have avoided our native bitter cherry (Prunus emarginata var. mollis) because the trees are all-too-often afflicted with Northwestern Anthracnose (Neofabraea spp., especially N. malicorticis).  I wouldn't want to spread such a mulch around apples, pears, and other rosaceous fruit trees which are highly susceptible to this infectious plant disease.  

I hope this helps a bit.  Good luck!

Reinettes.