Is There Any Way To Get Around Effects of Character Warping in Creation Mode?

The only constraint on accessories is which part of the body they're attached to. They'll move relative to that part's origin. Perhaps you can try attaching it to a different part of her body? If you're starting at shoulder, try starting at chest or arm and moving it to shoulder. If you're starting at chest and moving to shoulder, try starting at upper arm instead, or vice versa.

Failing that, perhaps you can move it out front a bit, so when her shoulder becomes inconsistent, there'll be a buffer before it starts disappearing. Of course, this means the flower would technically be hanging in midair, but from most points of view it should be fairly unnoticeable. If you have an extra accessory slot, you could try offsetting/hiding the gap with another accessory.

EDIT: I remembered this question and decided to try it today. I anchored a flower to her shoulder, and it followed the anchor perfectly, but disappeared into her shoulder when I attached it to chest. If shoulder's not anchoring for you, can you list what settings you're attempting to use? Everything on the "adjust equipment" screen if you can.