What a great question!! Thanks for asking it.
I expect (hope) they'll have the good sense to go around it - it's not usually very big - and to keep a sharp eye out for any aerobatic activity in AND AROUND the box. (Some of us aren't good enough to always stay in the area like we're supposed to.) That includes the upper and lower bounds of the aerobatic area. I, for example, start inverted spin demonstrations at around 10,000' AGL. Lots of folks work right down on the deck. Going over or under the area might not be the best choice. The best option for a pilot in transit is to go around and monitor whatever frequency applies for the airspace in question.
[begin ramble]
It's not that the aerobatic aircraft have the right-of-way, we sure don't, and it's not that the aerobatic pilot's see-and-avoid responsibilities are diminished in any way. But the reality is that it's very difficult for us to see traffic when we're working in "the box."
Keep in mind too that most aerobatic airplanes are very small and agile. Just because you have adequate separation one second doesn't mean you will the next. I gain about 1,500' in a hammerhead. It takes me approximately six seconds from pull to pivot at the top. That's a rate of climb of about 15,000 FPM. We've all seen pilots make 160kt 180 degree turns in less than a 1,000' radius. Often maneuvers involve pushing around a turn, or a rolling while turning. Many of us have periods of gray outs and poor vision at high g's. Add to that the fact that the acro pilot is pretty focused on what she is doing and see-and-avoid might not be #1 on the list of things to do. In other words, the odds of me seeing you come into the box when I'm flying a routine are just about zero.
The way I try to counter that is to clear the area before I start a sequence. But during a four minute sequence another airplane can easily travel 8 to 12 NM, meaning the airplane sharing the box with me at the end of the sequence was probably not seen when I cleared the area.
If I run into you, we will share the blame for our failure to see-and-avoid. But I have a parachute and you don't, so I'm the only one who will be around for the FAA to punish.
[/end ramble]
Chip