Very nice. I have a server side script called QuIP which is similar. QuIP is descended from PurpleSlurple, which I also created. Neither is particularly robust, but other's have extended the idea c.v. Slurpler, and further discussion has resulted in some related work c.v. xPunt. After PurpleSlurple and during the development of QuIP I too came across Donovan's Ahoy, but like Purple, Ahoy is a tool for the publisher. I wanted something for the consumer, so I created PSAhoy.
Look for lots of support and inspiration on the Yak list. I don't know whether you know it but you are in good company pursuing the granular addressability issue. Quotation in part[8.1] has interested Ted Nelson for decades. Purple and the other scripts mentioned above were inspired by the work of Nelson, Doug Engelbart (and others). PurpleWiki might interest you also as it supports transclusion (a related concept), as will, I believe, xPunt.
I agree that functionality like this should be natively built into browsers. I'm aware of one attempt in the past and some recent work to do this. Work by you and others will make this come to pass in the fairly near future I believe. In the mean time I continue to evangelize the idea and try to connect inerested parties and programmers.
Keep up the good work.
Matthew A. Schneider
 QuIP: http://sasites.com/quip/
 PurpleSlurple: http://purpleslurple.net/
 Slurpler: http://www.concept67.net/perl/index.html
 xPunt: http://www.concept67.net/blog/?page_id=141
 Purple: http://www.eekim.com/software/purple/purple.html
 psAhoy sample: http://sasites.com/suse/apache/files/psAhoy.php
 Yak: http://collab.blueoxen.net/forums/yak/
 Granular Addressability: http://www.eekim.com/software/purple/purple.html#nid06
[8.1] TRANSLITERATURE™: http://transliterature.org/
 PurpleWiki http://freshmeat.net/projects/purplewiki/
[10,11] These references elude me at the moment. To be continued...