This is probably something neither you, nor the font developers can do alone:

I am currently working on a dictionary with a large number of combining diacritics, sometimes multiple on a single letter. This need to be tweaked to look nice. I realize this has to be done in the font, and then be supported by rendering engines (such as Prince)

Example of such characters can be found in this document:

http://www.gutenberg.ph/previews/Franck-A.pdf

http://www.gutenberg.ph/previews/Franck-A.html

(I am happy to see OpenType support listed in your roadmap)

Graphite is a font rendering tool developed by SIL International. Stacking diacritics is something it does well.

http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&cat_id=RenderingGraphite

Graphite handles:
Diacritic Placement
Contextual Shaping
Ligatures
Reordering and Splitting
Bidirectionality

And the good thing is that it is free. Hope Prince will add support for Graphite.

Today I received the following:

Jim Albright
Wycliffe Bible Translators

Curious you mention it here, as I am working with the nice Gentium fonts from SIL to do this dictionary work.

Once we have fully supported OpenType layout in Prince then we may investigate Graphite also to see what additional functionality it can support. One disadvantage is that fewer fonts are using Graphite technology; perhaps only the fonts released by SIL?

Released fonts
Unicode encoded fonts
Abyssinica SIL - G
Andika - G
Apparatus SIL - G
Arabic Script Unicode Fonts
Charis SIL - G
Dai Banna SIL - G
Doulos SIL - G
Ezra SIL Hebrew Unicode Font
Galatia SIL
Gentium
Padauk - G
SIL IPA Unicode (beta)
SIL YI
Sophia Nubian
Special Interest Derivative Unicode Fonts - G
Unicode BMP Fallback
Fonts in Development
Tai Heritage Design Review - G

http://scripts.sil.org/cms/scripts/page.php?site_id=nrsi&cat_id=FontDownloads

You may want to switch to one or the Graphite enabled fonts as Prince will be supporting Graphite in 7.1. Gentium Basic does support Graphite stacking diacritics.

What do you gain with Graphite?
* Stacked diacritical marks: The placing of more than one accent mark above a letter.
* Context shaping: The changing of a letter's shape depending on its position in a word or on the characters around it.
* Reordering and splitting: The changing of vowels' positioning and shapes depending on the consonants around them. This element is especially common in Indian languages such as Bengali.
* Bidirectionality: The inclusion of different directions for writing. In Arabic, for example, letters are written from right to left, but numerals are written from left to right.
* In addition, smart font technology also makes the inclusion of different languages in the same line or paragraph easier.
http://www.linux.com/archive/feature/52884

Jim Albright
Wycliffe Bible Translators

I think all of those features are also available in OpenType, and in Prince 7.0 we have already added support for bidirectional text layout, mark positioning including multiple marks if the font supports it, and Indic shaping, using OpenType features.

You are doing a great job making a great tool. Keep up the good work.

Jim Albright
Wycliffe Bible Translators