Park crowding issue that are severe enough to limit or curtail guest entrance into the park are extremely rare. However, New Year's Eve is one of the times when you could possible encounter this.
When crowd control does happen, it occurs in phases. This is described very well by Moms Panelist Bret
HERE.
In the rare instance when there is a Stage 4 crowd control in place,
that means that no new guests will be able to enter the park. That
means no guests, not annual passholders, not DVC members, not people
with dining or BBB reservations. If Stage 4 is in place, the park is at it's
full legal by-order-of-the-fire-marshall limit. In this situation, you
would be unable to enter the park to go to the Boutique. If you end up in this position, you should call BBB reservations (407-WDW-STYLE) as soon as possible and
explain the situation. There are no guarantees, but my
understanding is that in this rare situation Disney would be lenient on the cancelation penalty.
My advice to you would be if being in
a certain park, meeting a certain reservation commitment, or dining at a certain restaurant on a holiday is a
priority for you, plan to be there as far in advance as possible. I personally would plan to be in the park in the morning, even if your BBB reservation is in the afternoon. I'd rather not take the chance of disappointment.