Yes, this is definitely possible to do. The reason why the existing combo packages have such a small selection of speeds is that, unlike the DVD-ROM firmwares, I haven't written a patcher for the combo firmwares, so I'm still doing everything by hand, and creating a dozen different versions with different speeds (the 9S1B package, for example, contains 20 different speed variants) without a patcher is just tedious.
This has been on my to-do list for a while now, but there're so many other items on the to-do list that I just haven't gotten around to it.
I'll see what I can do.
If you don't want to wait and want to do it by hand... then use the fc command to find the 4 differences between the 12x and 16x patched firmwares. These four different bytes should represent, respectively, +RW, -RW, +R, and -R.