Hi Crogge
To answer your question I just install FMRadio on my N900 HongKong PR1.3, plug the wired headphone.
I notice there are two version of this: 2010.03.20 and 2010.03.60. I guessed the later was the latest so I tried it.
FMRadio 2010.03.60 just simply crashes to the desktop.
A reboot won't help.
Without removing this version, I tried to install FMRadio 2010.03.20, but was anable to install.
So I uninstall the 2010.03.60 version, install 2010.03.20 version, and it works without a reboot.
I tried to scan to frequencies lower than 87.5 but NO, it can go no lower than that or higher than 108.0 MHz.
I can not hit any Japanese broadcast with it anyway, neither on PR1.2 nor 1.3.
I tried installing FM-Boost but it still does not help.
After a reboot with both FMRadio and FM-Boost installed, nothing change: frequencies ranged between 87.50 and 108.00.
I do not install Japanese locale on my n900 though. I just use UK locale with GMT+9 for time zone.
Hope this post does answer you