No problem Shy.....one shall never go much below 0' AGL no matter how badly it turns out!
If one starts at 4,000 feet AGL....that gives you 2,500 feet before it becomes an issue. One would assume the entry altitude would vary by aircraft type....something like a Bell 47 would differ than say....an H-53E. The Demo is only to show Incipient VRS....not entry into full VRS.
What rules apply to such exercises in CAA/JAA/EASA or whatever-you-are calling-yourselves-this-week-land?