Let me sum it up, if i got it right or not.

1) if user specified no charset/encoding in JBCL URL
2) then Jaybird assumes that user did specify "NONE"
3) and then Jaybird overrides this user choice by OS-specific and locale-specific default
4) and effectively assumes that charset WAS specified to - for example - WIN1251
5) then Jaybird connects to Firebird server assuming charset connection to be WIN1251 (or other OS and locale specific)
6) but reports to server that the connection is in "NONE"

Is it so ? de facto using one charset and de jure reporting another ?

If so then i am completely at loss, how is that rational choice.
Use whatever you like, even LATIN1 but do report your assumption in effect to server so server could adapt itself to your workings.

I truly hope that i misundertood something, otherwise i have a gooseskin.