Is this a feature request or do you want to try and implement what you have sketched?
What I see looks possible, using sub summary layout parts and portals, but there are also simpler options that don't take as much effort to produce.
Yes this is a something I would like to implement into a solution. So any pointers would be much appreciated.
It takes a bit of creativity in terms of relationships, sorting the relationship, inequalities, a calculated key, but it can be done. here's a demo file. It's not fully complete as it's just a proof of concept. You'd need to set up a script to manage the number of records in Directory, their "Letter" values and their Sequence values to make it work without a lot of "hand management" of the records in that table.