Wouldn't easily work, as the Pi2 PCB isn't really suited for a device the size of a Pandora / Pyra.
You'd need a customized PCB which has the keyboard, etc. included, otherwise it would just make the device huge and bulky.
Just think about it: Minimum thickness would be the USB / network ports. But you'd need to add a battery, a keyboard and a PCB for the keyboard, etc.
The Pyra isn't that expensive because of all the parts (okay, the CPU is more expensive than the full Pi), but because it is customized.
With all that customization, etc., such a Pi2 would be a lot slower than the Pyra and approx. 100 - 150 EUR less (even with higher production numbers, unless you are talking about a production of 100k units).
Of course it would need additional board for keyboard attached to PI. The size/bulkiness wouldn't be so bad, here are some photos:
The position how it could be done - hdmi and audio would need to be rerouted by cable extensions to top (hdmi) or upper left (audio). Red rectangle is keyboard board attached to gpio on the left, blue is space for battery.
The thickness at usb port is 2cm (red), thickness of the rest of the Piu board where the keyboard board would lie is 1cm (green):
and here is how it looks in hand, it could still be a bit wider than twice the width of PI case, it is 9x12,5cm.
When I hold PI inside case (which has lot of empty space) in my hand it does not look bulky.
Of course one custom board designed directly for the device will always be better but this is about compromise and reusing existing solution to save the price and piggyback on Pi success and big community.
So instead of whole device this would still mean custom case, keymat, lcd, keyboard board with lipo charger and controller chip (some arm cortex like in Teensy or Maple) sending key events and doing power on/off. So it looks like most of custom costs are still there but the board is much simpler and existing PI OS/software would be easy to reuse so one could save on the development. Also if it would be sold as a kit/docking station maybe it would not need some certifications (wireless) or extra fees (sd, hdmi, codecs)? If this could be made to be sold for 200EUR it would make sense to me. But if you say it could shave off only 150 EUR (from 500-600?) it would still be very expensive indeed.
Of course it would not be as good/fast as Pyra but the quad core with the GPU and other cores the PI 2 has is still plenty for emulation and much better than Pandora. And there is lot of optimizations on the software side and agreements with various third parties done by Raspberry Foundation which would bring additional value for such device.
But again this is a bit off topic here so lets stop or continue elsewhere.
EDIT: As for cost, here is something similar but a bit larger
https://www.indiegogo.com/projects/pi-top-a-raspberry-pi-laptop-you-build-yourself#/storyThey estimated it to 299USD including the pi board, and larger batteries/lcd but not custom keyboard, only the case.