They won't allow *you* to walk thru the park in your wedding attire, but your guests will be OK. However, people have had various levels of success getting clearance to have their guests meet the guide (no charge) at the park entrance instead of at International Gateway. Also, it is not possible to break up the hours of the chartered motorcoach over the course of the day. We had to pay for 2 five-hour minimums in one day b/c our ceremony was in the morning and DP was at night.
However, there are alternatives to the charter that are cheaper, including one-way buses, vans, limos and mini buses. If most of your guests have been to WDW before and are comfortable using the transportation, you can skip it altogether. Then either you walk in with them and a guide, or you get a limo just for yourself (if your planner says your DP location is too far from International Gateway to walk in your dress).