I've figured out how to connect non-HID (ie, SPP) bluetooth keyboards! It turns out that once you establish the initial bluetooth connection (ie, with rfcomm connect /dev/rfcomm0 XX:XX:XX:XX:XX:XX), you need to run an intermediary driver binary called kbdd, which can handle a fairly impressive range of non-HID keyboards. I wasn't immediately able to find a pre-compiled binary for Freemantle, so I went ahead and compiled my own, which you can download here. Usage is extremely staightforward, but be sure to read these instructions anyway.
Now on to applying the correct layout settings! (The keyboard may be connected, but I'm now having the same keymap-related woes that many of the rest you have reported).

Many thanks to those who helped me get this far!

EDIT: It turns out that this svelte little daemon package may also help with many of the keymap problems that people have been running into - because if you download the kbdd source, you can actually code specific keyboard layouts directly into the compiled executable. I know this because the supplied profile for my particular keyboard (a freedom model) was actually somewhat incorrect, and I discovered that the most effective and painless way to fix those errors was simply to go into the source code and move keys around. Granted, this approach only works (a) if someone has already set up a base profile for your model of keyboard, or (b) you have a fairly sophisticated understanding of the physical architecture of your keyboard, but I figured it was worth describing here nonetheless.

Anyway, the gist is that I now have a non-HID Freedom keyboard running pretty much flawlessly. Thanks again to everyone who made that possible!

Lots of thanks to Qole; I was able to modify his instructions to get x2x working with all the normal keyboard buttons to share my laptop's keyboard and mouse.

I did have to make one change; enter wasn't working quite right. I had to press it twice on the terminal, and it didn't work at all in some other programs. Editing the pc file under symbols (both copies) to change KPEN to Return and RTRN to KP_Enter (to match the default rx-51 file) fixed that, though.

Is the "dev_ ..." the mac address for the N900 or the keyboard?

If N900, mine is 20 : D6 : 07 : 55 : 39 : E1 *, which doesn't quite fit with the format given in post #161.

that should be the BT keyboard's address. you can get it's mac via "hcitool scan" (as described by JoHnY here).

yes it does those "xx"es are just placeholders. every mac address has the same length, twelve bytes. (six pairs)

btw, all those commands were single commands split by a backslash ( \ ). using a backslash, you can write a multiline command, by "masking" the linebreak with it.
JoHnY did the same in his post (manually connecting the BT keyboard), he just wrote every command in one line.

I also have a spp keyboard. But it appears the kbdd file and the instruction link are both broken. Does anyone still has the binary and the instruction? Thanks in advance.