Yes, I know about the "INT" bit in DMA list entry description for sure, but it's unknown if it was ever supported by any actual C65, and AFAIK Mega65 does not support it currently (?? at least I don't emulate that in my Mega-65 software emulator). But about plans in this direction, Paul can say a lot mode than me, of course.
Please note, that c65manual.txt and similar files are hard to tell how accurate, often (even at F018 DMA controller description) it's stated that some things are different or even features not available at "initial versions". Moreover, I think, we know at least about two very incompatible F018 revisions, F018A and F018B even the DMA list length and the layout is different, and some known to be existing real C65 ROM images use on or the other only. Who knows, maybe even more variants, after all, existing C65s were only prototypes in various stage of the development in different state. But still, it's true, that besides this issue, there can be extension in Mega65, especially when it does not conflict with too much C65 software already exits, just because there are not so much of them :)