The put agreement is also filed by Ambac on the SEC EDGAR site.
It looks like they are rated AA btw.
There is no question about the Trust being liquid as it holds short term commercial paper. That is not the issue. The risk is Ambac seizing the cash to pay creditors ahead of the preferred stock in a Chapter 11 situation.
I would also be curious if your broker will give you a bid for the bonds.
It could all be moot if Ambac never exercises the put and continues making the bond payments. I don't think it is costing them much more than 1% annually to do so and I think it would be a tremendous black eye if they screwed the bond investors by exercising the put. Therefore I think it unlikely outside of a Chapter 11 filing. But Ambac filing Chapter 11 is not out of the realm of possibility.