[NTG-context] \MPpos, pxy, initialize_box(), oh my!

hello all, i finished a presentation (my first ConTeXt creation!), and now i've got a couple days to get creative again. i'm trying to understand how the typeset lines of text in the MetaPost manual were outlined in Section 5.1. i've just discovered that \MPpos only seems to work in MPpositiongraphics, and initialize_box sets `pxy' to \hpos's contents bounding box. what if you want pxy to be not strictly a rectangle, but the path that encloses text, etc., which is just part of flowing text in a paragraph, like page 247 (beginning of Positional Graphics chapter) in the MetaPost manual? i've read the MetaPost manual. i hope the answer isn't too obvious. thanks, adam