your onyx is a peak detecting charger in NiMH mode so yes, the TX will block it.
There is a way of modifying the tx so it doesnt but its fine desoldering/bridging where a diode used to be... not for the faint hearted or inexperienced.
best way to use the onyx is like racerx said with a 8xAA battery holder plugged to a normal charge lead, or the plug mod to the radio's battery tray. The charge socket on the TX will not work with anything but the most basic, dumb, fixed rate chargers, unless you're prepared to open up the TX and do electronic mods