My guess is that you'll quickly find the pyramid shape of the freewheel bodies to be frustrating to work with, but it might work.

There are, of course, 8 speed freewheels, but that might require changing the dishing, or going to an off-center rim. Or, perhaps using an 8 speed freewheel as a donor.

Any chance of using a 7-speed freehub, and building a cassette, 7 of 8 (for 8-speed) or 8 of 9 (for using 9-speed parts)?