Supporting Arabic mark placement independently of top/bottom anchors

[yanone]

This is not necessarily a ufo2ft issue, maybe more a usage question, as it may be supported already.

I want to place Arabic tashkeel marks independently of top and bottom anchors in some cases. For narrow glyphs with dots (like noon medial) the mark placement on top of the dot using the dot’s top anchor may look okay, but oftentimes for a balanced and natural appearance, the tashkeel should be placed independently of the top/bottom anchors.

What is a good way to approach this? In an ideal world, I would only create one addition mark_top anchor where necessary, either in a composite glyph or in a base glyph, with the anchor in the composite glyph taking precedence over the base glyph anchor.

Can I write my own filter for this? Is this already supported somehow with specific anchor naming? In my above idea, the mark_top anchor would connect to _top anchors automatically even though they are probably not automatically pairable in today's code. But what is thinkable, if I want to keep my sources clean and maintainable, is to add my own preprocessor that adds missing _mark_top anchors in the location of existing _top anchors, so that no code changes would be necessary for pairing.

How would I approach this?
Also, it sounds like it could be related to Simon’s previous issue #591. Is it? Is there a priority issue here?