Given it's probably accomodation, this can happen:

1.) before check-in, without card present (prepaid rate or cancellable rate with the cancellable option lapsing): http://www.flyertalk.com/forum/europ...sion-2.html#19

2.) at check-in, when card details taken and the hotel opts you in for express check-out: http://www.flyertalk.com/forum/china...on-56.html#829

As well as 3.) check-out, in a traditional card-present transaction.

Consent is superfluous. After all, here in HK where I live (and Singaporean FTers report the same for Singapore also) it is up to the cardholder to prove he opted out, no matter what the rules (VIOR) say (bank refuses to act on cases where opt-out is proven, and regulators will not compel banks to act unless they are not acting on proven cases).

This is shxt hard for (1) and (2). You really have to persuade the bank to void the card not present charge/express checkout arrangement.

(3) works differently in UK given terminals ask you the cashier for DCC instead of ticking. But even if somehow applied by cashier error, rate will be displayed on the transaction slip and should be a big red flag to ask the cashier to void (if possible) .

With hotels, I'm routinely using my Standard Chartered HK Amex (with a approx 3 Asia Mile/USD earn rate) for prepaid rates and deposits.
Rate might be crap but not for many currencies (GBP and AUD only) and I get a decent earn rate and ease of mind.
Quite hard to figure out whether my spending is qualified spending for other HK Visa/MC promos anyway (normally requires a card-present or an internet purchase transaction, rather than a transaction charged by hand like (1) above).