Recently an evolution of the Asterisk 1.8.x build that I submitted a ticket for at OpenWRT SVN has been accepted to OpenWRT trunk.
I've since then decided to recover my build environment and rebuild 18.104.22.168 using what landed upstream in trunk for backfire on ar71xx and brcm2.4. I've also included the WIP patch for invisible support from http://www.personal.psu.edu/wcs131/blogs/psuvoip/2011/01/asterisk_hack_make_your_google.html
Note, invisible support is shared across clients because it's a proprietary google extension to Jabber. If you set it on Asterisk, you still won't be able to be shown as visible from a regular client like GTalk for Android or Chat in GMail.
I'll try to keep an eye on https://issues.asterisk.org/view.php?id=18727 to see when it's properly included so that I'll switch to the patch upstream instead though.