I have very mixed feelings about 'shame attacking' techniques.
What most people are anxious about is not negative reactions per se; what they're anxious about is being judged negatively for who they are. If you go somewhere nobody knows you and pretend to be someone you're not, those people are judging a persona you've created, not who you really are deep down inside as a person.
I suspect a person can tolerate a good deal of negative judgment when they know that it's not really them who is being judged. You can see this sort of thing happening to some degree even when people aren't pretending to be someone else; many people feel less anxious around complete strangers they never expect to meet again than they do around people who know them. That's because they don't expect anything to come back to haunt them. Many people can do things on the job they can't do in their private lives because they're representing someone else, not themselves. I could make 50 phone calls a day at work without thinking too much about it, but I'd still have to spend hours psyching myself up if I had to call someone on my own phone representing no one but myself.
I think for a technique to be truly effective it has to expose the real you to the judgment of other people. But I've never looked into the research about this particular tactic.
Good thing I read other people's replies first because this is exactly what I thought upon reading the original post.
If the therapist is anticipating that someone is going to go out of their way to be aggressive to a homeless person then this would be quite irresponsible of them to put you in that position, both in therapy terms and for your personal safety. But more likely, the expected reaction is that passers-by would either give you money, say they don't have any, or just ignore you. If the latter is what was meant by "public being rude" then I'm not sure how effective this would be towards overcoming rejection.
By the therapist's own admission, his recommended plan basically relies on people's perceptions of homeless people, and provides an alias that faces rejection - it's not the person directly being judged - and this is what I've noticed with most of these shame-based exposure techniques.
To highlight this point further - a more effective rejection exposure method would have been for the patient to walk around in a nice suit asking for random strangers for money, because the public is not used to this scenario. And even this involves hiding behind an alias.
Not trying to be harsh. But perhaps its better for the rejection exposure therapy to focus on scenarios most reflective of what the person normally faces as possible - along with assessing the impact of their rejection anxiety on their ability to actually face it. But I'm definitely no expert on the matter.