Most attractions already the right amount of ride vehicles on the attractions, the only time you need to add another vehicle is if you have to remove one for problems.
Not only that, on most attractions adding another ride vehicle would actually cause the lines to get considerably longer, since the ride would have to be completely shut down for the vehicle to be brought out. Since there would be a desire to not break up the ride experience of those already inside, that means that either everyone in the ride would have to get through first (which would mean that no one in line can load during that time) or the entire line would be cycled through (which means that no one else would be able to enter the line). There are a few rides, like Test Track, where vehicles can be brought in or taken out with the rest of the ride in operation, but they are in the minority.
The on-site manager or lead will know when he needs more cashiers, and if as you say someone somewhere knows where additional staffing is, the on-site manager/lead can call that person.
Since I no longer work at WDW, I haven't seen what, if any, changes this system has on day-to-day operation, but I am sure that the center for line busting has access to the staffing levels throughout the entire park, something that local managers would not necessarily have, since it wouldn't be considered essential to their role. (At Disney, if something isn't deemed necessary for a CM to have access to, they won't so that there is less chance of it being misused or accidental mistakes being made.) I remember how many people were surprised that, as a Modified Work Coordinator, I had access to all schedules that were in the Cast Deployment System at all locations in the labor region (even World of Disney NYC, since that belonged to the Downtown Disney area!). However, it was because I needed to be able to modify schedules for CMs for the day that they entered Modified Work (and sometimes other days) by cutting or deleting their regular shifts to enable me to enter them in our area. As our CMs could come from any part of our labor region on any day, I needed to be able to do this with any area (except obviously WOD NYC, but that came part and parcel with access to WOD at Downtown Disney). In most cases, their managers or coordinators back in their home areas would have to call the Deployment Base to do the same thing, since it was not a regular function of their roles, except between locations in their own area, which they could do on their own.
Also, on the topic of overstaffing, I think that extra staffing, the other shade of this that you mentioned, would be more appropriate. There is a level of extra staffing in place in most areas to cover contingencies, plus, in the middle of the day, there is usually some overlap between CMs who are about to go home and those coming in to work, so that the area doesn't become understaffed. Additionally, during really busy times, an increased number of CMs on Modified Work are usually scheduled to assist Guest areas, especially retail, in backstage capacities in order to free up the regular CMs to help on stage. (A certain number of CMs on Modified Work would do this, anyway, at any time of year, but the busy periods will see many of those helping non-Guest areas scheduled to Guest areas during that time.) You usually won't see them, but they are there helping the area.
In the event that an area does have too much staff, they will usually offer the option to Early Release and, if needed, also send home any CMs on Extra Hours shifts. On any given day, there generally are enough CMs who want to leave early for one reason or another to cover this need, should it arise, especially near the end of the day. Those that don't want to go home will take turns being tasked to do things that need doing within their areas. In most cases, the sharp and sudden variety of overstaffing occurs because the actual attendance is considerably lower than the projected attendance, which determines the scheduling. In one case, I was supposed to be on (I believe) a five or six hour Extra Hours shift at the Main Street Bakery during one of the Mickey's Not So Scary Halloween Parties. However, it rained that evening (and not the typical 15-minute Florida rain, but a more sustained rain for about two or three hours), so many non-party Guests left the park earlier than the closing time and fewer Guests than expected came to the party, which wasn't sold out that night, anyway. (I assume, though, that the Guests did start trickling in later after the rain stopped, since they wouldn't want to waste their tickets. By that point, though, my shift would have ended, anyway.) So, after I finished the couple of tasks that were found for me to do in the absence of Guests to serve, I was sent home after only two hours.
