Can a sealer be used in a fireplace, firepit, barbecue, or other area that gets extremely hot?
That is not a good idea. Do not use a sealer, or other chemical product, where it can burn and release fumes.

For fireplaces, a better option might be one of the special "high temperature" paints that are made for an application of that kind. If dealing with a flooring (like brick) that runs right into the fire box, consider using a high temperature paint on the firebox floor and then out in a pattern that ends in an area of normal temperature. Then the balance of the flooring can be sealed as usual.

How hot is too hot?

Do not use a sealer on any surface that at its hottest, you cannot maintain hand contact for 15 seconds.

In floor heating systems:

They could not get hot enough to effect the sealer as you would not be able to walk on a surface that hot.