Can I have a "hollow" basement bathroom floor?
A good practice is to pour a solid concrete slab over the sewer pipes. Any exposed wood would eventually rot, especially if all you have under it is a dirt floor.

These installations are tricky. Common problems are that the pipes are not vented properly, they aren't pitched correctly, or the type and/or size of the pipes is incorrect. You should also make sure that there is at least one drain cleanout installed. This will allow access to the pipes if they become clogged.

Here is a guide which will show you the basic steps involved: http://www.familyhandyman.com/bathroom/remodeling/how-to-plumb-a-basement-bathroom/view-all