DIMing an array to -1 is just my way of indicating that the array has no dimensions.
Be careful with this one ...
Dimensioning an array to -1 elements will actually (at least in my experience) make the array invalid within LB until it has been dimensioned at 0 or greater again. So if you attempt to access any element of that named array, you will get a Subscript Out of Range error. This is technically not a documented feature, but I utilize it in my Dynamic Array Library. For similar reasons which you have mentioned...
Check out the code below. On Error GoTo [Error] Dim myArray$(10)