I did some investigation and attempted to add this feature.
The good news is that there are no conflicts with the roles we currently use. According to the W3C specifications the caption element can be used with any other ARIA attributes. The only case where it shouldn't be used is when a table is used for layout purposes and not to display data; that is when it has the 'presentation' role. The following pages are used for reference:
The only issue is when we have scrolling and staticHeader enabled, since in this case we render two <table> elements: one for the headers and one for the data. So, the <caption> element would naturally go in the table that actually displays the data. The problem is that, because this caption text is rendered on top of the table, this breaks the flow with the header table, and the table as a whole doesn't look as good. We might say that we could simply render the <caption> element in the header table, so that the caption appears an top of the whole ACE data table, as we know it. However, this could be misleading for people who actually need this feature, as the header table contains no data. So, we might simply go ahead and implement this feature, even if it doesn't look as good on tables with scrolling and staticHeaders enabled, or we could simply not supported in such case.