The Fostex FE87E has a small cone and modest x-max. On an open baffle with no protective highpass filter, the math says it starts to go non-linear at about 7/10ths of a watt at 200 Hz, and at about 2/10ths of a watt at 100 Hz and below. In other words, it will theoretically start to lose clarity at fairly modest sound pressure levels when used as you describe, though it may be designed to "gracefully" go non-linear and thereby give you a few more useable decibels.
If you size the baffle correctly, you can electrically high pass the Fostex at between 400 and 500 Hz and use the dipole hump to extend the acoustic response down to 200 to 300 Hz. The baffle does not need to be huge, probably about 20 inches wide. Then add a 15" high Qts woofer (0.7 to 1.5) to the baffle that electrically crosses over at about 100 to 150 Hz which is extended up by the dipole hump to meet the Fostex. If you use a sealed or ported sub you will need to go higher in frequency before crossing over to the Fostex. The displacement requirements for the Fostex would be controlled by this crossover configuration. Try using BASTA to size the baffle and crossover.
I have done this and it works well. Good performance at a reasonable cost.
@unsound and others interested, check out the OB/Dipole Sub offered in kit form (only) by GR Research in association with Rythmik Audio. Similar in design to that of Linkwitz, but with superior 12" drivers, H-frames, and the Rythmik Servo-Feedback circuitry. Ideal for use with planar loudspeakers as well.