Lello,
Just been taking a crash course on the FZ30. The only thought I have is a very low powered flashgun mounted on the hotshoe, Page 97 of the manual, and using this to trigger your other flash guns provided camera does not try a preflash through that . If you have a flash that operates at less than a trigger voltage of 24volt you can give it a try.
Set on camera flash to manual minimum power and slave flash to maximum. If the shot is seriously underexposed then camera is pre flashing through the hotshoe gun as well as pop up. If exposure OK or overexposed then a weak hotshoe mounted unit can be used as a trigger.
The best non internet deal I have found is Jessops 100M. This has a low guide number of 10 for £10:99. As most pop ups have a guide no of around 15 this is a nice weak unit to act as a trigger. Should not affect exposure unless the camera is sitting on top of the subject.
Don