Planning a pipeline

My team is working on a continuous pipeline of work rather than a single coherent project. I’m struggling to figure out how to get Mingle to give me useful predictive information. Does anyone have any ideas?
We are maintaining and extending a mature system. Work comes to the team either as small projects that consist of 10-20 stories which take 2-4 iterations to complete or individual requirements which are often broken down into a handful of stories.
The stakeholders prioritize a single backlog of small projects and individual requirements. We would like to be able to tell them when we expect to finish the high priority work from the backlog so we can have discussions along the lines of “well, if we do project A and project B before project C then it won’t be done until the end of April, but if we can push project B down the priorities then project C could be done in the middle of March”.
We have modeled the project-story relationship as a tree. With other teams I’ve used a similar structure for requirement-story or epic-story. The point is that it’s helpful for medium-term planning purposes for the stakeholders to prioritize at a larger granularity than the story.
I can think of two views on the data that would help this, but there may be others. I don’t think either of them is achievable with the features Mingle supports at the moment.
The first is a burn-up chart where the total scope is limited to a subset of projects (just high priority ones). Then you can use trend lines to estimate when you will be done. However I don’t think it’s possible to filter the cards included in a chart by a property of their parent in a tree (I suppose what you need here is the opposite of an aggregate property—an inherited property, perhaps?). So the only way to achieve this is to manually update the priority of every story rather than just updating the projects.
The second would be a table or graph showing cumulative capacity given some predicted velocity alongside a total size for high priority projects. I don’t think you can do this kind of spreadsheet stuff in Mingle.
A related issue is estimating the size, for planning purposes of work that hasn’t been broken down into stories or for which the stories are not completely estimated. If there are story-level estimates you want to use them aggregated. I suppose for this you would need to be able to provide an override value for an aggregated value in a tree. Does that even make sense?
Ben