Certainly they don’t officially support it, but it ought to work. Just about anyone who uses or has used TrueCharts has run their NAS behind a Traefik reverse proxy. SCALE runs just fine behind Caddy as well, without any special arrangements or configuration settings. No idea what’s going on with Synology’s reverse proxy, but it definitely isn’t a simple matter of “TrueNAS doesn’t work behind a RP.” A common issue I’d see cited is support for websockets, so OP should make sure that’s enabled.
Now, whether this is a good idea is another question–and I suspect it is not. But it should work.