The crux of the problem
This is a bog-standard problem seen over and over in the DIY stack, when obsolete/cheap dimmers, motion sensors or other smart devices are used with efficient, driver-based LED lighting.
Older switch loops do not have a neutral wire. To solve this problem, older/cheaper, obsolete dimmers, motion sensors and other smart devices use a characteristic of incandescent lights - their extremely low impedance when off. They leak their operating current through the "off" incandescent filament. This doesn't work on LEDs; they are so efficient they turn all current into light.
The proper way to solve this problem is bring neutral to the switch, and have the smart switch power itself between supply hot and neutral. Like a normal load.
The direct solution
The problem with your idea is you're not allowed to horkle-dorkle "hacks" like this where mains wiring is concerned. You must follow your local Electrical Code, which says every device must be approved for use in mains wiring, and be installed according to its instructions.
Here's what you can do: a relay that is approved, such as an Aube or RiB. You wire the hunk-a-junk dimmer in series with a relay coil, with the coil returning to neutral. Separately (it can even be off a different circuit), you take supply hot to one relay contact and connect the other relay contact to the lamp(s).
Now, the relay coil may be too high impedance for your obsolete sensor. If so, you can add an approved "dummy load" in parallel with the coil, such as the Lutron LUT-MLC, which is approved for this purpose.
It acts like a resistor. Actually, it's a capacitor tuned for your mains frequency.
Or, just fit the dummy load directly across your lights
Like I say, dummy loads like the Lutron LUT-MLC are designed and approved for the purpose of shunting across LED lights to provide an alternate current path to get obsolete dimmers to work. So you could just fit this directly to your lights. Problem solved.
This method will work no matter what, even if you have an old-school "switch loop" which has no neutral at the switch. Never misuse safety ground for neutral, it defeats the purpose of having a ground wire.
Indeed, one of the diagnostics we give people who report this problem is "Replace one of the bulbs with incandescent, does the problem go away?" The incandescent acts like the dummy load, or to be more precise, the reverse.
Or get a modern motion sensor
Get a motion sensor that has a neutral wire. It powers itself via the supply hot and the neutral. It places no demand on the bulbs at all, and will in fact work properly with no bulbs connected.