I know that this is a bit off-topic for the board, so Steve please forgive me, but this has me thinking.

As a thought experiment:

I wouldn't use Pi-Star. I would create my own install of MMDVM and MMDVM_HS. Configure MMDVM to work as normally, but use something like iptables to block incoming UDP packets from the network. This may result in what you want - a 'hotspot' that hears, but can't talk.

I say not to use Pi-Star only because there's a lot of overhead for functions that are being purposely broken for this application. Pi-Star also has auto-update features that can cause your desired config to change without notice.

-Steve KC1AWV