News: IMF IMF Bentham announces funding of class action against against UGL Ltd, page-12

I full agree with your speculation. I think it would be a much more straight forward case with regards to continuous disclosure obligations of the company and investors purchasing during a period when the market was not fully informed on material information which has subsequently been proven to affect the value of the company. So it's a pretty simple argument for economic loss, as no investor would have purchased at those prices had the company fulfilled its continuous disclosure requirements.

For holders prior to those dates, they would have to somehow prove they would have been one of the first out the door before the SP dropped, if material announcement was made. In reality, a trading halt would have accompanied the announcement and would have ensured a big gap down in SP, so I wouldn't like their chances. The only other course of action might be to assume there should have been more regular updates from the company which would have been progressively worse and allowed the more risk averse to sell out (prior to full SP drop).

As IMF is about getting justice at a fair risk/reward price, then I'm glad they are only going for the low hanging fruit. I'm sure there will be plenty of revelations and whistle blowing during the court case as people try to save their reputations/finances that may enable the pre-existing holders to make their case afterwards (hopefully funded by IMF if the risk/reward is justified).