I'll second what's already been said, and add that a ND filter can be quite useful in situations like this.
Bottom line for me is that if you want to take a quick snap in challenging lighting, then the modern TTL stuff (particularly, IMHO, the stuff from Nikon) is simply magic. Works a dream.
If you want to create a more composed and deliberate photograph, that's easily done too, but it takes more time and care. The essence of the problem is always that the ambient scene has higher EV than the subject. If you can't knock that down with a high shutter speed (you say you are locked in to 1/90s, but the same problem exists with any focal plane shutter camera), then a ND filter is a good option....if you have enough flash power to overcome it on the subject.