This looks to be the never ending story...
I've been out today and had a look behind the door panels again, and can inform that the depth mentioned earlier in the thread is not entirely correct. With the woofers in the same position in relation to the door/panel, the available depth is about 98mm in the front and about 120mm in the rear (less in the front because of a support for the window). This opens up to about every 6,5" speaker set available. Need to mention though, this will include a lot of modification, fabrication and adaption. The goal will be to modify the BOSE brackets as they seems to fit a little bit better for the purpose than the std brackets, this will include reusing the "back cover" so no water or dust will have an easy access to the rear of the speaker element.
The ring mentioned in the previous post is so wide (185mm) that any average 6,5" will go inside it, so not cutting of this would be necessary I think. If needed, the woofer could be mounted about 15mm further out than original position.
At the same time the panels was off, I had a closer look at the rear door tweeter, and it's actually the same as the std front tweeter! Not the same part number as it didn't have the extended mounting bracket as in the front. This will be a massive upgrade!
I've been thinking to go for same set in the front and the rear, but has lately reconsidered to go for better quality in the front and a bit less in the rear, as I'm never in the back seat anyway.