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OK, let us continue the discussion about FEN here, then.

To Fergus:
What you use in Game Courier is indeed very close to what I had in mind.
In particular allowing a multi-character representation of a piece by
parenthesizng it. You use braces {}, I proposed normal parentheses, but
the idea is the same. You also allow for variant-dependent meaning of the
single-letter codes.

The main question that arises when defining a FEN format that can be
applied as universally as possible over all variants, is which letters
should have predefined meanings under which conditions. And in how far
these conditions should be recognizable from the FEN itself.

I would be inclined to pre-define as few characters as possible, using
only the board size as an indication of which default assignments are in
force. For 8x8 these would be the standard Chess pieces, for 10x8 A and C
would be added for Archbishop and Chancelor. For 9x9 we would have the
Shogi pieces (S=Silver, G=Gold, L=Lance next to PNBRK, where the latter
would now refer to the Shogi version of P and N, while B & R would be
subject to Shogi promotion rules), and for 9x10 Xiangqi pieces. Crazyhaue
and Shogi would use the postfix-'~' and prefix-'+' conventions for
promoted pieces.

This covers the most common variants. Perhaps Shatranj is a bit
short-changed, because it shares 8x8 with Chess. To fix that we could add
E and F for Alfil and Ferz to the 8x8 default set. I tend to think that it
would be unwise to try pre-defining pices for other variants, as it would
not be possible to do it for all. So I would prefer treating all other
variants symmetrically, and do it for none of them. This would mean that
all other pieces would have to be described by a multi-letter code. Some
variant of Betza notation could be well suited to this, as for the more
common fairy pieces this results in quite short names.