After playing around in HornResp for awhile, I came
up with a tapped pipe with the following specifications:
The total length of the pipe works out to 177+11+11=199 cm. To convert this into an actual box, I divided this by four, then decided on dimensions that would result in a cross-section of 290 cm^2 for each section (here's where I made my first error - this approach actually results in a pipe length that's shorter than the design target, as it does not take into consideration the effect of the 180 degree join at the end of each section!).
HornResp predicts that the frequency response of this tapped pipe would be as follows:
The graph suggests that the frequency response would be essentially flat between 43 Hz (the length of the pipe works out to 1/43 the wavelength of this frequency) to just above 100 Hz. There's a minor peak at 135 Hz, followed by a huge dip, but all of this is happening well above 80 Hz, the usual upper cutoff frequency of a bass unit or subwoofer. The "hash" above 200 Hz can basically be ignored - I'm sure that the folds in the tapped-pipe are going to have a radical impact on the frequency response at that point.
Once folded up, box size works out to around 2.4 cu.ft., which is quite large for a single 6.5" driver. On the plus side, there should a lot of room in that box to mount the driver, without overly restricting the cross-section of the pipe.