I feel your pain, I wasn’t aware of the issue until it was too late to return the WAM750 I got. At least I got mine on clearance for less than $100. The tech support answer was the same to me… it is not a supported device and that only Bose was but as you also found out the Bose does not support notifications. I did test with Sonos by the way and they have worked flawlessly for me and not cut off the announcements.
I am interested in @bamarayne work around though because these WAM speakers are supposed to support DLNA.