MOSFET with load on source alternative
If I get the question, the idea is that the host will power off, and so take the GPIO down with it. And, you'd like to keep the sensor grounded.

It's possible to connect an N-FET as a source follower. The problem is, the source will be one Vgs below the gate. If you have a 2.5V threshold FET, and the gate is at 3.3V, then the output voltage is only (3.3 - 2.5) = 0.8V. Clearly, that's not going to work.

An n-FET with a weak pull-up followed by a p-FET high-side would accomplish this. This would use zero power when the host is off - exactly what you're looking for.

A non-inverting buffer like an LVC1G125 can sink or source 32mA. This would also work.