I'll need to check, but I recall sane-backends was compiled so that it should look in /usr/local/etc/sane.d for conf files - this being said, I know that hplip still insists on looking in /etc rather than /usr/local/etc.
For the sake of removing one variable from the testing, I would recommend putting the conf file in /usr/local/etc/sane.d and making a symlink /etc/sane.d -> /usr/local/etc/sane.d just in case
From your dmesg output, it looks as though 0x378 is the correct address for your parallel port, the following in /usr/local/etc/sane.d/plustek_pp.conf should work:
[direct]
device 0x378
Edit: I checked sane-backends and it does not appear to be compiled against libieee1284, so putting /dev/parport0 in the conf file would probably not work
Edit1: if you start xsane from a terminal window, do you get any error messages?