I have created a font that required an especially high Ascender and an especially low Descender (2335 & -1441) in order to allow for long strokes in a script style, handwritten greek.
The thing which concerns me here is that the Ascender is more than 2048.
I do not know the answer to your question about overlapping, but I am wondering if you might find a copy of my Chronicle font useful to look at, as I tried to produce the effect of characters with long ascenders and descenders.
http://www.users.globalnet.co.uk/~ngo/CHRONICL.TTF
The effect is that the font can look good at 72 point.
I am not saying that this was necessarily the best way to proceed: I wanted to produce the effect and worked out a way to achieve it.
Anyway, I mention this font in the hope that the trying of this font might perhaps be helpful.
William Overington