While the elements included in att.placement cover most of the things commonly occurring in margins or otherwise displaced from the normal textual flow, I wonder whether it would be completely inappropriate to also add <head> and the generic <seg> element to this class? Working with medieval recipe manuscripts, I frequently encounter recipe headings which are placed in the margin, as well as cases where the last few words of a recipe are placed in the margin (or some other place deviating from the general flow of the text) due to lack of space, and would like to be able to indicate this in a TEI-compliant way.
I realize the <label> element is intended to be used for marginal headings, but it has the problem of not being allowed within the <front> of a text. My theoretical approach involves viewing recipe collections as discourse colonies and individual recipes as independent texts, and since the title of the recipe thus logically goes to the <front> (along with a possible recipe number and any reference markers), the <label> element (which, by the way, seems to be rather sparsely defined and documented at the moment) does not currently work.
As to the annotation of stray bits of text that have been shoved into all kinds of available spaces, this is by no means an uncommon practice in mediaeval manuscripts. While my annotation is intended primarily for use as linguistic corpus material and I thus really cannot adopt the purely visual encoding developed for genetic editing (as much as it does appeal to me), one of my central concerns is also the annotation of visual paratext, including layout. For this purpose I have developed a simple system based on the use of @place, @rendition and @rend; the only problem at the moment is that I have no TEI-sanctioned way of indicating the displacement of individual segments of text (and headings) from the textual stream, for which the most logical solution would be to allow @place on the <seg> element.