I have the following keys working with only modifying the rx-51 file and rebooting:
fn+z=>tilde
fn+c=>pipe
fn+left=>escape
fn+right=>tab

The only two keys that don't seem to work are:
fn+up=>page up
fn+down=>page down

The lines that don't seem to work for those entries are:

key <UP> { type[Group1] = "PC_FN_LEVEL2", symbols[Group1] = [ Up, PageUp ] };
key <DOWN> { type[Group1] = "PC_FN_LEVEL2", symbols[Group1] = [ Down, PageDown ] };

Anyone have any suggestions? I also tried PgDown/PgUp, but that didn't work either.