How to interface a SPST switch with minimal current (1µA)?
If the latency (in reading the switch) was acceptable at once per 100 milliseconds then why not let the micro wake up for a short period (in as low a current mode as possible), energize the switch's pull-up briefly, read the switch, then do what it has to do then re-enter sleep mode.

The switch is only briefly taking current when the micro wakes up. 

Maybe the problem with this scenario is that the micro wakes up for 1 millisecond and consumes 1mA for that period BUT if it only does this once every 100 milliseconds then its average consumption is 10 uA. At least that may be better than energizing a closed switch continually?

As an aside you've got to take care to feed a switch with enough current to break thru oxide layers that form - it's called the switch's wetting current and some can be several milliamps.