You would have to fabricate one - would not need a large volume - maybe a liter unless you are planning to flood the diaper plain (EEK LEAKS) ! If doubled or tripled slit diapered with pads or liners the volume could be increased incrementally not withstanding the diapered waist band being able to handled the load before the whole she-bang goes PLOP PLOP and DROPPING off splashing with a THUD THUD !
A liter bottle or enema bag worn in back pack at chest or shoulder height would work with gravity feed.
Would need an on flow and off flow valve system and hose directed from that back pack either down thru the crack and up to the dispensing point or around the waist and down the rise to that dispensing point. Without a shut off valve - possible to put the fluid into an squeezable vessel and when squeezed start the flow but no way to stop the siphoning once started other than to pinch the tubing or remove it from it's source .
To avoid the gravity siphoning feed could put a tire valve in the mix and when pumped with pressure squirting the fluid out more rapidly than the gravity method. A remotely operated electric valve could cause the flow by the operator of the remote under no control from the victim - very interesting !!!
As for temperature - could fill the bottle or bag with hot water which in short time will assimilate room temperature below body temperature. Actually cold water would have an amusing effect and shocking when initiated ! This would not go unnoticed !
Another aspect to consider is having the fluid with a colored dye - when the diapered area has reached and overflowing it's containment the faux pissing will visually be displaying that color - EUW that victim (person) is peeing his pants RED (other color name here) if not the color of choice ! So don't let a fantasy like this go undeveloped !
The flowing of warm water in the groin area could induce unintended peeing - like some people can't stand near running water without the urge to pee ! Look out when that happens the diapered containment area will be stressed way past it limits and the flood gates opened. Thus if you can imagine it - It can be done in ways that can satisfy curiosity humiliation punishment and a whole nother level of entertainment !