I would like to be able to create custom spreadsheet functions that accept a 
range parameter that works across sheets.

E.g. In a cell on a spreadsheet:
=FNMYCUSTOMFUNCTION(Sheet1.B41:Sheet6.C88)

Function declaration:
Function FNMYCUSTOMFUNCTION(rng)

Currently returns: Err:504 Error in parameter list.

Note that =SUM(Sheet1.B41:Sheet6.C88) works as I'd expect summing all
the values on all 6 sheets between B41 and C88.

Note also that =FNMYCUSTOMFUNCTION(Sheet1.B41:C88) also works as
expected - passing a two dimensional array to the variable rng.

This enhancement would mean that custom functions could work in the same way as
the built-in functions. Separating the range into several single-sheet range
parameters is not a particularly satisfactory solution as spreadsheets using
such custom functions would be harder to maintain. E.g. Add a new sheet and
every instance of the function would have to be changed.