Since we will be trying to incorporate signature capture in an upcoming NT6 project destined for iPad use, we have started to research how that can be accomplished since it is not something which is typically inherent in web-based applications. What I have found so far are a few jscript products out there, here are two that we are starting to look at:
http://webification.com/signature-for-web-forms-jsignature and
http://www.unbolt.net/jSignature/ (same product)
http://thomasjbradley.ca/lab/signature-pad/ and a youtube video of this:
http://www.youtube.com/watch?v=TzdUgB4-UMsI don't know if they will be suitable, but thought I would share this on your post. The same concept for signature capture I think may work for your purposes of text annotation, etc.
Our mobile applications require customer signature, and so far we have used the standard 'electronic' signature where the client types their initials and we capture the IP address and time/date stamp (and now with NT6 we can capture the geocode as well), and usually that is considered a legal signature (at least here in the U.S.). However, our clients that purchase our software have their own thoughts on this, and really prefer the standard signature capture as if they were signing a piece of paper. So not one to turn away potential sales, we're looking to find a signature capture solution as well (and as you mentioned, not to convert to text but simply a graphic tool that allows a 'drawing' on a web-based app and then save it for later viewing). I'd be interested in hearing also about any other solutions or experiences in capturing signatures on a tablet with web-based apps (i.e. NT6). I think jquery is going to be the way to go to find a solution.
Bruce - perhaps this is something that your team may consider incorporating into NT down the road since for mobile apps it is a in-demand topic? In either case, if we are successful in implementing one of these products in the coming months I"ll post back here.