And what would be the reading on that barometer?
We could probably take into account the mass m of the manhole cover, the area of its surface facing the outside A, the earth's gravity g, and the air pressure P0 underneath the manhole cover before it is lifted. That way we can find the difference in pressure from outside the sewer to inside the sewer DP, that is just below the absolute lowest possible, and the pressure outside P.
So, perhaps it would be something like this:
DP = m . g / A.
P < P0 - DP.