Yes, it can be done safely, but the key is the type of battery you put under there. It has to be a sealed, maintenance-free battery. And because of the space limitations under the seat, most batteries will have to be placed on their side, which pretty much dictates an AGM battery, and an Optima is exactly that (albeit a sponge hybrid and not a true deep cycle battery).
Do keep in mind that an Optima, Yellow or otherwise, is an AGM battery and thus requires the charging regimen of an AGM battery, and the Sprinter's alternator absolutely will not output the proper voltage to keep an AGM properly charged. An AGM connected to the Sprinter's charging system will be subjected to chronic undercharging, and will reduce the lifespan of an otherwise properly charged (and not too deeply discharged) AGM battery (Optima, Concord, Discover Energy, all of them) to perhaps 2-3 years rather than about 8 years (5-8, 6-10, depending).