Although you can't get features like this built into the web interface — at least not until FastMail either builds it themselves or allows extensibility so that a third-party developer can plug it in — there are alternatives to Boomerang that just use straight IMAP and would therefore work fine with FastMail.
Newton comes to mind off the top of my head, although there are others.
In most cases these services are designed to have you use their own apps instead of a web client, but the rules and processes are handled by a cloud service, not the client app itself, so things like "snooze" would still be applied to return things back to your inbox.
At the end of the day, however, this is why I still think Gmail is generally a better e-mail solution for folks who prefer to "live on the web" as opposed to using native clients. I use FastMail specifically because it has such great support for IMAP mail clients.