I was able to get Smart-ID working on GrapheneOS! Just went through the standard flow, nothing special. I had an existing Smart-ID on another phone and used biometric method(where you take photo of your id and scan the chip on it with NFC). Everything works fine, can use it to log into my bank.
PS: Sorry for bumping old discussion, but it appears as top result in google when searching for Smart-ID and GrapheneOS.