Scott Rodriguez wrote:
Looking at buying an altitude encoder for the transponder in my
yet-to-be-born homebuilt.

I see some altitude encoders have an RS-232 output for sending altitude
information to a GPS. What kind of functionality does this provide? Is it
"real-world" useful?

It's required for IFR approach certification I believe. I'd suspect
that the MX20 also uses it for the terrain warning feature because it
asks me for the altimeter setting periodically as well (which only
would be useful if it was looking at a pressure alt somewhere).