I think you are right it seems to be the perfect poison pill.
I have seen this before with other transactions. Seems to work well. In the past there had to be special language to keep the pill in effect, basically the language is that the shares become a joint venture for voting purposes only between the lender and the preferred's holder.
This keeps the leverage in place but insures the lender that there will be no dilution by the current holder to diminish the banks position. In case of a hostile move the lender and the company move in unison.