Fly as a capital ship
There is a trick that FREDders can do while FREDding a missiion that can allow them to pilot ships that they normally can't. This includes freighters, cruisers, capital ships, and even the Colossus.
For starters, suppose that you have 2 ships in a mission. Alpha 1 is in a Loki, while Beta 1 happens to be an Orion. Save the game, start FreeSpace 2, go to the mission simulator, and select the mission. When you're there, go to ship selection and there will be 2 icons, Alpha 1 (you) is a Loki, and Beta 1 is an Orion. Switch those 2 ships and start the mission. Congratulations, you're flying an Orion now.
The other way is to table hack to make them drivable.
1. If your using FSO and sent Beam cannons unlocked by default in the Mission SPEC, all beam cannons will fire normally, except for yours. To make your beam cannons fire normally, you have to set a Beam-free-all SEXP on yourself.
2. Warping out while flying a capital ship is quite different from NPC capital ships warping out. Instead of shooting out forward at incredable speeds, you will be going at a slow pace, and the subspace vortex will take a minute to show up, as will take another minute for you to enter it. Mission designers should keep this in mind when requiring the player to flee a supernova in a capital ship.
Is this worth it?
The answer is no. While it may seem cool to control a massive ship, keep in mind that you will be going slow, turning is horrible, and you have no primary gun mounts. Although you have dozens of turrets to do the work for you, having AI controlled turrets doing all of the shooting for you can really get you bored very fast.
Additionally, as your subsystems are damaged, the damage list will grow until it obscures most of the screen.