I do this every now and again, using a pencil held in front of me and released once 0 G has been achieved. I then see how long I can 'hover' the pencil for(normally a couple of seconds). Everything else is secured prior to the exercise.
It generally reuires no more than a 2.5 G pull to recover from a 30 degrees nose down condition.
There is nothing wrong with a bit of fun, provided it is executed safely.
As far as pushing 1/2 or one negative G in any aeroplane goes, if you think this will stop an engine or introduce a risk of fire due to fuel travelling out of vent tubes: Get Real! Do you really think any aeroplane would have a design fault as dangerous as that allowed?