Really like this idea - they could start with their own shares if they do another round in 2020!
There has been some previous discussion of similar ideas including a secondary market for unlisted shares, which included a great in-depth post from Calum (not sure if he is still with freetrade, but the points should still apply). It does sound sadly like it would be a lot of work/risk, so perhaps something for the long term, and maybe the initial crowdfunding would be doable