Heh, first post since ages.
If you only count the default RAM swapped in (from $8000 to $FFFF), it amounts to 32Kb of memory space. Some of these areas are reserved for OS use, and that is how you get the 24K-and-something that shows up in the MEM menu.
Depending on your hardware version, another couple of pages are available. On some versions of the 84+-line, you can access up to 128Kb. Wikiti has information about those pages. I haven't touched my calc for a long time, so I probably wouldn't even remember the port numbers.
Also, the OS technically doesn't "know" about those pages, hence whatever data you store on them will not be available for "normal" calculator use.
Permanently storing data (i.e. anything that requires a longer shelf life than your runtime buffers) in the reserved RAM areas is a bad idea, because they will very probably be overwritten.
So no, there's no good way to use that extra RAM. You're stuck with 24576 bytes.
EDIT: grammar.