Not easily. But see:
Create an executable file <Choices>/MIME-types/application (or the type you wish to use mailcap for) with the contents:
see `rox -m "$1"`:"$1"
'see' is provided by the mime-support Debian package. It may have a different name on your system.
You can then copy/link this to whatever mime type you wish. If you link it to all of the media types (application, audio, image and text I think) then mailcap will be used instead of Rox's Choices system. I don't know if Rox has any support for a fallback media type.
Most of the MIME-types scripts I saw used $@ and this uses $1.