When you mount a partition, you're effectively adding it in as a new directory (or to replace an existing directory). I'd suggest checking under /opt to see what directories are using a lot of space, which you can easily replace. Either use "du" from the command line, or the Storage Usage app to look at this.

For example, on my N900 /opt/maemo is about 320M, so that'd be a useful one to replace. I'd need to make sure that it's mounted at the right time though, otherwise any libraries/applications in there would be unavailable which could cause problems booting. Apps like stellarium or MAME would be simpler to replace, but they're only 35-40M each.