I don't know if it helps but my setup is atm broken (edit only for one N900). Maybe the bluetooth dbus device has changed. When I try to connect like before I getting now an audio connection.

As far as I remember:
N900_1: not working cssu testing maybe other testing stuff from bt
N900_2: working cssu stable some packages from cssu devel but not related
Have another one with stock but not tested atm.


Here my notes(which where working in march 2019 and today):
Device must be paired once and should be trusted.
(a note I made to the test-network script "changed Network1 to Device1" However try first without modification it is not everytime necessary) This was a hack which was needed because the N900 was identified wrong and which seems to not working anymore due to some applied system updates? Will look into it maybe we need to change the bluetooth class on the N900.

needed packages on PC:
bluetooth
bluez-test-scripts
bluez-tools

PC as root:
bluetoothctl
power on
agent on
pairable on
when you need pairing (look for mac): scan on
when you need pairing (pair the device): pair 00:00:00:00:00:00

PC as user replace with your N900 mac address(if input/output error do again):
cd /usr/share/doc/bluez-test-scripts/examples
./test-network 00:00:00:00:00:00:00 nap

in genwall:
when the bt icon get blue click fast the start button which resides on route-bt tab(settings on that page should be saved before and select dnsmasq to forward.)

PC as root:
dhclient bnep0

in genwall:
in genwall setup the firewall and forwarding you could do it also before making the connection.

{local-basics}
1.:select wan device gprs/usb/wlan
2. I would "accept connection from local machine"... makes live easier (really this should be activated!)
3. If you want to connect from pc->N900 without openning ports. I would also recommend to "Accept connection from local lan". (or allow port for dhcp)

{for/out}
4. activate forwarding and set lan device to bnep
5. Masquerade wan device should be active
{local-basics}
6.gen rules
7. push the start button


Disconnect:

PC as user:
ctrl+c
PC as root:
pairable off
agent off
power off
quit

EDIT: Oh on another N900 my tutorial is still working

And btw with genwall you could also make a wep wifi hotspot in addition with a firewall, also with your own rules.