Okay, here'* my extremely crappy attempt to draw your idea. I think it should work, good plan! This will also boost the lows while the highs are on giving you extra light. You will retain factory brightness levels while on standard low beam.
Let me try to decode this crap for you.
Everything in black is factory wiring, everything in red is added on wiring or the relay.
HL = low beam headlight, pos and neg terminal labeled (+,-)
HB = high beam headlight, pos and neg terminal lableled (+,-)
R = relay
D = diode
i = input signal for the relay
o = output from the relay
I didn't show the power and ground connections for the relay, but that'* a given. Also this is just for one side, another wire from the output terminal of the relay should go to the positive lead on the other low beam headlamp.
Let us know how it turns out, and if I missed anything!