Found where I got my Linux-for-Zire:
I ran Linux (OE) on my Zire 72 back in the day. As you say, not quite useful; slow and with few utilities (and running X was a bit overkill on that platform). But I could say to my nerdy friends that I ran Linux even on my Zire. And now I can tell my nerdy son that I ran Linux on my Zire. And in time I will be able to tell my nerdy grandson that I ran Linux on my Zire.
The point of this tail is that it is clear that a thin keyboard that can snap into place is well withing modern engineering and manufacturing specs. I see no reason that a manufacturer could not build a keyboard that snaps into a slide out case. Cases are cheap to make and manufacture. The manufacturer could make a single part that would be snapped into and plugged into cases that they manufacture for various phones. This would reduce the engineering and manufacturing of the expensive part of the product and leaving the cheap part of the product the part that gets customized.
If they wanted to get fancy, they could also include a battery pack, and a passthrough microUSB charger, so that when using the bluetooth keyboard, the phone would double or triple it's runtime.
No doubt not everyone would want one of these, but by spreading the cost out to dozens of phone models, there is likely large enough demand to make it worth while.