But something like this did happen when ASICS were first announced. Although this wasn't done by an issuer, all mining assets tanked.
Even Gigamining was down about 50% AND stayed that low for over 2 weeks. If gigavps really wanted to, he could have taken advantage of the clause at that time and everything would have been "legal" so to speak. I know giga wouldn't do this, but he certainly could have.
There is no advantage to take. The mining bonds prices went down because the market expected ASICs to arrive and correspondingly the coupons from the bonds to decrease. Assuming the market is right, the issuer's debt on the bonds went down anyway, buyback or no buyback. The buyback clause in this case does not cause the investors to lose or the issuer to gain - the loss/gain was due to the hardware advance.
There just has to be a better way, a better worded "out clause" for the maintainer that is fair to the investors as well. Because the scenario above can still happen (especially since the block reward halving is coming).
Once again - anything which affects the profitability of mining causes the bonds to decay whether there is buyback or not. Block halving in particular is a known event and should be priced in to the bond valuation in advance, if there is any drop in the traded price around the time of halving it means the market is irrational.