While things like IV hydration and antibiotics are easily worked out with the hospice team, hospitalization for something related to your terminal diagnosis is not.
The Medicare Hospice Benefit is paid for by Medicare Part A, which normally covers hospitalization. Medicare won't allow a hospice agency and a hospital to bill Medicare Part A for care delivered to a patient for the same diagnosis at the same time. Private insurance companies use the Medicare benefit to model their own coverage, so the same is usually true for patients with insurance.
The important factor here is the terminal diagnosis. If you are admitted to a hospice with a diagnosis of cancer, for example, Medicare wouldn't pay for any hospital visits for things related to your cancer. This may include pain, shortness of breath, dehydration and weakness. If something were to happen that was unrelated to your hospice diagnosis, for example, if you fell down and injured yourself or your pre-existing heart failure worsened, Medicare would pay for your hospital visit the way it normally would.
Hospice can be revoked at any time, so if you were on hospice and decided to seek more aggressive care, you could call the hospice agency and request to discontinue care. You could then go to the hospital for any reason, and Medicare would foot the bill. Since hospice isn't really designed to be a revolving door, though, it's best to wait until you are ready to let hospice deal with any symptoms that come up. You may be surprised at how well hospice professionals deal with symptoms and crisis situations in the comfort of your own home, without the hassle of a crowded hospital.
As I mentioned before, other issues, such as IV fluids and antibiotics, can usually be worked out with the hospice agency. These things aren't right for every patient, but hospice will discuss your individual needs if you have concerns.