Gary,
On a superficial analysis (which is all I can do, since I don't know which specific tool you're talking about) I estimate that a JavaScript solution would be much more costly (i.e. much more demanding on the servers) than a native solution. As far as I can (theoretically) tell, a JS solution would involve a juicy piece of AJAX -- as such, it would have to make entirely new, unjustified requests toward the server in order to find the previous edit on the user's talk page. Had this been implemented natively, the whole of that request could be replaced by a single SQL query. But then again, this is only an outsider's assessment, maybe things are more/less complicated than I'm estimating.
Cheers, Gutza
Gary Kirk wrote:
There is something like this in one of the popup scripts on the English Wikipeda; it may be Lupin's - I'm not sure - which shows "all changes since your last edit". Perhaps this could be adapted or implemented as you suggest.