What about the idea of allowing the public to address the house from the public gallery by prior appointment? Eg PP reps could apply in advance to sit in the public gallery during a debate on the Patents Bill, and participate in the debate. Maybe at least one MP has to nominate you, and their can only nominate one person/ org per bill, so there can't be more than 120 extra people in the debate (which would get messy).
Another idea: what about a rule change allowing parties/MPs who do have seats in parliament to give their proxy to someone else, issue by issue? For example, let's say Party X (8 seats) decides they support the PP approach to information freedom issues, and recognise that PP has a better technical grasp of these issues. Whenever an IT-related Bill passes through parliament, MPs from Party X nominate PP members to sit in their seat, participate in debate on their behalf, and cast their vote.
I think it would be better to have full-time MPs because otherwise it may be difficult for the best suited people to have time to also essentially do the work of an MP. Each bill goes through several readings, and if there are several relevant bills it could be tricky to arrange time off work to get to Parliament every time, and then they have to keep in touch with the people they represent, keep up with all the relevant information, and figure out how to best explain it to others (which is a considerable art). If we have full-time MPs, then they don't have to try to fit this workload around their existing jobs (and hopefully fit in time for their families too), because doing this would be their full-time job. And we wouldn't have to fund raise for their travel and accommodation each time (and we shouldn't have to, funding for MPs come out of the public's taxes, and should be allocated fairly, as should representation generally).
In any case, the review is only about MMP specifically, not other aspects of how Parliament/Government works, and I think these suggestions probably fall outside that scope (also, I think new ideas were supposed to be raised in the initial submissions, and this phase is supposed to just be discussing what's already been put forward, although I could be wrong).