You are the first to ever ask for this, curious what your use case is. I believe this can be done from the CLI using dumponchaindescriptors command. You'll need to SSH into the server and access the CLN container. https://docs.start9.com/0.3.5.x/user-manual/container-access
this is the sort of shit that makes Start9 fucking awesome. specific, detailed support of edge-cases over nostr. if you aren't supporting them you should be. nostr:nevent1qqspgv6hs726dl8pn9we52efjsverdxdlgps7ettls893advsuudvpspr3mhxue69uhkummnw3ezucnfw33k76twv4ezuum0vd5kzmqzypt2dd6nw0y00wfu2w72apkcl7at48e2rvupygz5lndh7wlkgkmjwqcyqqqqqqgrz4pkw
Based on experience, I have to copy out the hsm_secret and use this tool to capture the xpub https://github.com/dieqohc/HSM-to-XPRV-recovery-tool