Setting attributes in the subject, like [nosy=+foo], messes with threading in 
gmail. It would be nice if one could also set fields in the (top of the) body, 
e.g.

title = foo
nosy = +foo

I really like this idea and will try to implement it. Some discussion might help me:

Would it be acceptable to mimic the nosy-email format? That means fields below
message, after a '----------'.

Regardless of fields placement, should there be a marker/tag to explicitly
delimit commands? I ask that because, e.g., your original message could trigger
this behavior when you only wanted to discuss it.

Should the attribute-setting part be stripped from the message?

Should setting attributes both in subject and in body be allowed in a given message?