Sorry it took so long — I somehow failed to notice your request to add support for Sylheti. I fully enabled it now. I see that you made a lot of translations already, and I hope that you can continue.
Do you also need a keyboard layout? Will you be able to make a pull request at GitHub for jquery.ime? The instructions are at https://github.com/wikimedia/jquery.ime/tree/master/rules , and you can copy rules from similar languages. If it's too complicated, I can try to help.