I have a Lumotec N plus, which I believe has the same guts as the Oval plus. I experimented with putting an LED flange bulb in it a few years ago and I can confirm that it works. I ran mine with a bridge rectifier to convert the hub's AC to DC. The LED bulbs have their own driver circuitry in them and I don't know how much it likes to be reverse biased (if at all!), so I think a bridge rectifier is needed. Without one you'll get light half the time when it's switched on, but the other half of the AC cycle, when the polarity is reversed, you won't get anything and there's a good chance you'll fry the bulb's driver.
I can't see any harm coming to a taillight connected to the headlight, especially if it is LED-based with its own standlight. It just takes a portion of the current. If the front bulb failed, I'm pretty sure (but not 100% sure!) that the taillight has a Zener diode or something to protect it from overvoltage.
The Lumotec plus's standlight powers a small LED, so there shouldn't be any issue with a LED replacement bulb interfering with that. It presumably has its own rectifier.
I was disappointed with the light output from the LED bulb, and switched back to halogen.