I know in the non-Bose cars the rear side-panel speakers don't get a full-range signal. That may be part of it. You'd have to do some measuring, but it might be possible to mount shallow-depth 8" free-air subs in place of those stock speakers. For example, Audiofrog GS8ND2 subs are made for these kinds of fitments. The
Five Star Car Stereo video of a 2017 Camaro install is worth watching. Starting at 12:13, they show how they fit 7" Kenwood speakers in there. Just going by those visuals, I think 8" could be made to fit, but you'd have to get subs that are specifically made to work in a free-air fitment (no sealed or ported box).
I am not familiar with the Bose signal outputs, I follow your thinking about why they might have unused sub outputs on your convertible, but I wouldn't count on it. If they don't, you can probably tap into the front-speaker signals; run a low-pass filter, stereo-to-mono summing device, and line-output converter to a dedicated sub amp in the trunk*; and run that output to both sub drivers in the side panels. That assumes that the front dash/door speakers get a full-range signal like they do in non-Bose applications. If they don't - which is very likely - then you are going to need a digital interface like 17rsvert discusses above.
*This doesn't have to be big or expensive - the Kicker KEY500.1 should work well and combines all four of those functions (summing, LOC, low-pass filter, and amp) into one small box.