I've created a Colemak keymap file for Haiku (should work for old BeOS also), and submitted it: https://dev.haiku-os.org/ticket/3944
Unfortunately not all dead keys are supported (only acute, circumflex, diaeresis, grave and tilde dead keys are defined on BeOS).
But all special characters directly under the AltGr key (in Haiku: Option key), like €, «», ¿, ħðþ, å, ç, ß etc, are supported: