Regarding your request for proposals for typsetting sequence diagrams ("I am waiting for proposals on how sequence diagrams should be typeset in MetaUML. Code snippets help best.").
I stumbled over this http://www.martinfowler.com/bliki/UmlSketchingTools.html
which references this http://www.spinellis.gr/sw/umlgraph/
I like the syntax a lot and it should be possible to implement something similar in MetaPost. What do you think?
Make sure to have a look at the documentation as well: http://www.spinellis.gr/sw/umlgraph/doc/indexw.html it has many very nice looking diagrams.
Thank you for your input!
I like this style of syntax too and I do hope that I will find time to implement it, but that won't probably happen in the next few months...