Can a short-term visitor to China sign up to Alipay (Taobao) or Tenpay (WeChat/QQ) mobile payments, without Chinese ID or a Chinese bank account? If yes, how?
Yes, and you do not necessarily need a Chinese bank account. You have a few options:
(1) you could add money into your Alipay account using mobile phone refill cards. These refill cards are purchasable at any convenience store or supermarket.
(2) you can get your identity verified with a non-Chinese passport, although it will take 24 hours (it's instant for Chinese Resident ID Card holders). Instructions are here. But 24 hours isn't bad in any sense, anyway.
(3) Alipay is usable at many restaurants and shops in China. I am not sure, but I think using the "pay by QR code scan" functionality requires you to have Fast Pay enabled, which in turn requires you to have a Chinese bank account. If you're just shopping on Taobao, then you don't necessarily need a bank account & UnionPay card.
(4) If you decide you don't want to open a bank account, then you probably wouldn't benefit much from being able to use Alipay. However, any place that accepts Alipay would very likely accept cash, so just bring enough cash with you. And most places where the expense is too big for cash would usually accept major international credit cards.