Ah, just a note, when you get to it, the reply window needs (unless you don't want) icons for yt vimeo and 3d model links (that's how I learned a bit of BBCode without having to bother asking people).
Also, for sigs, how about a faded double arrow pointing downwards at the end of a short, fixed length sig that will expand only when clicked? Make the double arrow button clickable area the same width as the post, and have it unobtrusively highlight itself on mouseover.
Basically, thin clickable rectangle the width of the posts, of a slightly lighter color than the current HLP background color, that becomes visible when mouseover, with only the small grey double arrow being visible when not in mouseover (perhaps have the double arrow change to black in mouseover to stand out more against the clickable rectangle).
.... does that suggestion make any sense or am I not being clear enough?