It doesn't sound a very good idea! - but you don't give many details for us to base a judgement on. Exactly which capacitor in what sort of power supply is it?.

Photoflash caps are electrolytics. But they're not usually rated for continuous duty. I'm not sure if you can compare the capacitance and voltage rating on the package as being equivalent to a general purpose electrolytic. Probably not a good thing to try.

I wouldn't think there would be a problem, providing the capacitor has a low enough ESR it'll be fine.

I know the exact model number of photo flash I have and they don't even list a ESR There's a reason for that, they're produced in bulk and cheaply with wide variances to reduce cost, so the individual device to be used would have to be tested in order to determine if it were okay for use under anything but short pulses infrequently. Even a badly manufactored capacitor is going to last a pretty long time when used at the rate that a disposable camera flash is. I've never taken any apart (they're expensive obviously) so I don't know what kind of capacitors higher end cameras use.