I have a proposal that has many lines. The formatting choice of 1pt is too heavy and detracts from the data and the "hairline" is too light. Is there anyway to add a format of 1/4 to 3/4 pt?

No.

Other than to use a graphic of a thinner line.

You might try 2 lines set to hairline right next to each other.

By definition a "hairline" is the thinest line that the output device is capable of rendering. Which means that the actual thickness depends on your printer.

I agree, it'd be nice to have a line thinner than 1 point.