I've played in places that do not allow jump cues. No biggie, just grab the lightest house wood off the wall (usually 15oz) and use it. Works great as long as the obstructing ball is at least a foot or more away from the cb. This ensures that the angle of the cue won't interfere or trap the cb as it's trying to leave the slate.
Here's some useful stuff I've discovered: A 5 to 7oz jump cue with a hard tip is like a magic wand for close-up jump shots. A little heavier than that (like 9 to 11oz) is good for shots where the obstructing ball is about 8in to 2ft or so away. This is where most jump cues are regarding weight.
So, basically, lighter cues work better for close-up jump shots. The farther away the obstructing ball, the easier it is to jump with a full length cue, especially if it's a lighter cue, like 15 to 17oz.
Take a shaft only, like from your break cue, and you should be able to jump a ball as close as a 1/2 inch pretty easily if you can stroke it quick enough and strike the cb accurately.
The cb weighs 6oz, so with a shaft alone (weighing about 3oz) you need a lot of speed to get the cb up, but the shaft will never interfere with the cb's trajectory because the 6oz ball kicks the lighter shaft out of the way. Of course, using the shaft by itself is not legal, unless you have a 40" shaft.
The official minimum cue length is 40".