The short answer to your question is "yes". Just apply a Group (Command-G) to the first masked object or group. Then it can be applied as a mask to the next object or group.

Yeah, I guess your symbol method might give you quicker access to the objects in some ways. I'm wondering how far you could take that... If you converted every component into a symbol first, perhaps you could edit each piece (both objects and their masks) without ever having to ungroup. The downside is, I'm not sure it would make things that much easier, as symbols don't update live on the canvas, last I checked.

I'm a little rusty on how to edit complex masked objects. You can definitely use the layer thumbnails to select mask vs. object. You may also be able to use Select > Subselect to delve further into a mask (without ungrouping) in some cases...

Between the ability to select thumbnails in the Layers panel, and occasional use of Select > Subselect, you've actually got pretty good options for editing a simple mask. Once you start nesting them, though, you'll probably have to ungroup.

The cool thing is, you've got options for nesting masks and working non-destructively. And grouping and ungrouping isn't a big deal, particularly if you remember the shortcuts for it (Command-G and Shift-Command-G). On the other hand, it might be nicer if Fireworks offered the ability to "unfurl" a group or a mask via an arrow, while still seeing the full effect of the mask.

I just tried out my earlier symbol suggestion and realized that while one can apply a mask to a symbol instance, it may not be possible to apply a symbol instance as a mask, which is a bit surprising. (At least, that option seemed to be greyed-out in CS6.) So my earlier idea may just be purely theoretical.

You might also find these extensions from Aaron Beall to be helpful:

http://fireworks.abeall.com/extensions/#Mask

http://fireworks.abeall.com/extensions/#Object