It may not be possible. A convenient opportunity is OK, even if you engineer it a bit. But the more effort is required on your part, the less appropriate it is.
It might be nice to offer a compliment in the situation described in the question, but I think it's valuable to bear in mind that it's not very important.
If, on the bus, I saw someone drop their wallet I would be very proactive and intrusive about getting their attention so that I could return it. A dropped wallet, containing money, IDs, etc., is important enough that it's worth breaching the convention to keep to yourself. There is a specific goal, a reason that it matters, and the interaction naturally terminates soon after describing why you demanded their attention (if not immediately).
Simply admiring a slogan or image on a shirt or accessory almost never rises to this level. Accordingly, making the compliment should only happen if you can do so in a convenient, casual way. If you happen to make eye contact, as mentioned in the question, by all means mention that you like the bag. If you want to position yourself such that eye contact is more likely to happen, that's fine too. But since the generic value of delivering a compliment like this is pretty low, it doesn't take much effort to be "too much".
The social conventions in effect on public transportation simply don't prioritize this kind of interpersonal contact as highly as other things (like keeping to yourself). That's not to say that you can't do those things (I've had conversations with strangers on the bus and train), but it'll be immediately noted as out-of-norm.
