I've attended many conferences over the years. As far as I can recall, unless I had to book the hotel through the conference (not what you're asking about) the hotel has never checked my conference registration status when reserving. You could say "I haven't registered yet, but I will." I doubt they'd come back with "Fine, call us again after you do."

It can to the conference's advantage to have as many people as possible register at their rate, if they've guaranteed the hotel a certain number of rooms in exchange for the rate and they're having trouble hitting that number.

However, the conference can find out who's in their block of rooms. As a conference organizer I used this information to find people who were going to attend, had booked rooms, but hadn't gotten around to registering yet. (The list of potential participants was short enough to check room bookings against it. The likelihood of anyone on the list wanting to be in that city at the same time for another random reason, and being desperate enough for a cheap rate at that particular hotel to use the conference rate, was small.) If the hotel or the conference room allocation is filling up, they could potentially find poachers and cancel their reservations or give them the option of paying the rack rate. Just because I haven't run into this doesn't mean it won't happen.

If you're planning to walk into the conference without registering, though, that would be more difficult. Anyone without the right kind of name tag stands out. Walking around the conference area without one is asking for a visit from the organizers and an invitation to register or leave. This would be, at a minimum, embarrassing.