Unfortunately, IPNS over PubSub is not setup to be that special/different compared to the standard IPNS over DHT. It’s a reasonable feature request though.
In the meanwhile, what you can do is have a script that every so often calls
ipfs get /ipns/<IPNS_KEY> in the background. The IPNS to IPFS resolution should be instant because PubSub will have the latest result waiting for you, then it’s just a matter of fetching the IPFS data.
Alternatively, if you don’t want a polling approach and are willing to do a bit of work you should be able to subscribe to the IPNS over PubSub channel for your key and listen for updates (you could use either the
ipfs pubsub CLI or HTTP API to do this). When you get the updates you could call
ipfs get on the data. Note: if you do this that the IPNS over PubSub topics are not named the same as the IPNS keys themselves, but you can use a tool like https://github.com/aschmahmann/ipns-utils to figure out the conversions for you.
Edit: It looks like you’re looking for something like https://github.com/ipfs/go-ipfs/issues/4435 to be implemented. While this issue is old some of the IPNS improvements have put it on the radar. Unfortunately, it’s not the highest priority item for the core team right now, but IPFS is an open source project that welcomes external contributions so anyone is interested this issue is available