I buy a bag of shredded broccli stems (they market it for brocoli slaw) for my chicken soup. My girls still call it chicken noodle becaust the broccoli strands get translucent and soft. could work for Pho.
Oh wow... I forgot I had done a search, didn't realize how old this was. Oops
I've made pho with kelp noodles, and it's awesome. One of the benefits of using kelp noodles, is that the leftover noodles stay in tact for the next day, unlike rice noodles which fall apart.