On the same page that Michael gave there is a "Flash Variables" library that does exactly what you want - allows you to use unused Flash for variable storage. The only downside is that you need two spare flash blocks.

However, before you can use it you will need to adapt the IAP library (on that web page) to your compiler. If you open the rx2iaplib.a51 file you will see all the assembler for the library. You need to change the segment declarations, parameter passing conventions, segment naming conventions to match your compiler.

Andy