If you click ‘Parent’ on the post you wish to moderate, you can then moderate the posts inside that sub-thread without immediately losing any moderator points, if you then go back to the home screen (by clicking on the Slashdot logo) after you moderate the comments.
It appears that the system does catch up with you periodically, and then deduct the points you spent — but during that period of time it seems you can go quite far into negative deficit. For example, I ‘spent’ my last moderator point today five times (strictly for research purposes; I don't plan on doing so again. That would be bad.)
Checking again, I'm indeed now out of moderator points, but my 'over-mods' seem to have stuck. Perhaps others with moderator points can help verify this bug?
Although, I really wonder if this is a 'new discovery', or if this 'exploit' is already commonly abused... But that's just my cynical side showing through.