I know about their context menu entries but I'm curious if there's a way to have buttons for direct scan?
You can make buttons to run most context menu commands, although how well this works depends a bit on the way the other tool has implemented its context menu.
NOD32 unfortunately does not assign a verb to its context menu, so it has to be run by label. That's still OK, as long as the label doesn't change (and you don't change languages, etc.).
For details, see the top of ContextMenu command for how to use ContextMenu SHOWCMDS to generate the appropriate command for you, which can then be put in a button.
Some antivirus also have command line interfaces for initiating a scan, which are easy to turn into Opus buttons. NOD32 has one but, unfortunately, it does the whole scan in a command prompt instead of the nicer UI, so you're probably still better off using the first option.
I don't know the details of MalwareBytes, but if its menu shows up when you right-click files in Opus, or if it has a command line interface, then it should be easy to make a button which runs it on a file.