Interesting policy. Can't say I support it at all. But that's life. 
There are litterally squillions of people who can and will use IMP to 
send emails with special characters not supported by current easy to 
use copy past dialog box, including myself (the Esperanto characters 
seem to work fine in emails, and in IMP, I just have to paste them 
from another source to use them for example).



Sure, some people won't be able to view them, but your target audience 
always will ... they share your language and interests after all ...

We only show characters in that tool that are part of 8-bit ASCII to 
make them available to all translations, no matter what charset they 
use.

On the special character input window:



1) The Euro currency symbol would be very useful



2) The Esperanto characters would be useuful. They are six in number, 
being c, g, h, j, s an u with a diactritic (a ^ for the first six and 
a u for the last). They appear on Esperanto websites globally so it's 
conceivable to support them, it would be good.



This is them:



ĉ ĝ ĥ ĵ ŝ ŭ

Ĉ Ĝ Ĥ Ĵ Ŝ Ŭ



Should be easy enough to add to the "Other" list.