fbreader180 does work on 360+
The non built-in version of fbreader does work on the pb360+ -- but you need to use the version for pro, because the plus has the same architecture. All you need to do to replace the built-in version is put it in /system/bin/fbreader.app.
You might need to change extensions.cfg to use it first, instead of adobeviewer, depending on how that was set up. On my 360+ Adobe viewer was first in the list of readers, ahead of fbreader, so you may need to change that or open the file specifically with fbreader.
I've attached the extensions.cfg from my plus. If you need to modify it, you can change it and put it in /system/config.