I mean… I grew up in, and currently live and work in San Francisco, so I have no idea what a foreigner looks like. I imagine my response would be different because there’s almost nothing a person could do on BART that would cause me to want to strike up a conversation with them short of a direct invitation. That said, a T-shirt that said “I’m studying English, let’s have a conversation”, if the train weren’t packed, and if I didn’t have anything else pressing to do (like WK reviews), I think I might say “hi.”, especially if they smiled and were friendly.
But all of that is outside of the cultural context of Japan, where I’ve been told a bunch that the sight of white people evokes test-anxiety and a complete loss of ability to speak anything other than broken English. I’m not an expert on Japanese social order, so maybe someone who has been living there longer could say…
A better comparison for a visually perceivable difference, rather than language might be something like… a person on BART with a missing arm. Seeing something like that would probably give me a degree of anxiety, and I think I would (for any reason) be less likely to start a conversation with them, much less about their missing arm. But if they were wearing a shirt that said “I’m doing a study, ask me about my missing arm!”, and they smiled at me when I saw the shirt… honestly of all the scenarios this sounds like the one in which I would be most likely to start a conversation. I would push myself to participate because of my liberal guilt, much as a Japanese person might push themselves in an effort to be more worldly/cosmopolitan.
If that all is missing some piece of relevant Japanese cultural context, don’t hesitate to correct me. I want to understand the objection, but it all seems to boil down to “well, it’s just weird, isn’t it?” or “wouldn’t it be off-putting to you?” to which my answer is inevitably, “OMG I am the complete wrong person to ask about ‘normal’…”