These are not unrelated problems, and might have a partially related solution.
With 3 PMQs left before dissolution of Parliament, how about Ed Miliband asks the 3 big broadcasters for 2 questions each, which he will ask over the next few weeks.
The extra head to head TV debate can then be Cameron’s PMQ answer (already filmed), followed by Miliband’s answer to the same question. Given the number of PMQs that Miliband and Cameron have had so far, I’m sure there will be some previously “answered” questions that could also be included for a greater variety of topics.
If the broadcasters don’t want to broadcast it live (it’s not exactly BBC1 in primetime), post the video to YouTube and see what the national interest is in an online only debate All the other parties will then produce their versions of answers to the questions, and “The Internet” can then splice the various versions together for people who want a full 7 party debate.
If nothing else, it’ll improve the quality of the PMQ podcast for a couple of weeks. If it doesn’t and Cameron just ignores the questions, and Miliband’s answers are good, it might be fun enough TV for the broadcasters to run it…
(improvements on the idea welcome in the comments)