I actually have one of the sensors that you have on your car. It can be removed and replaced with the standard low coolant sensor for the S4 PN 8553-15-610B . Then just replace your filler cap with one that has the overflow nipple from a 87 PN N326-15-17YA.

 

That is a great idea.

I have a spare S5 coolant level sensor (no overflow line) laying around, so I would just have to change the connector over (according to Mazdatrix it is a simple male/female bullet connector). That should remove the hose coming from the sensor, thereby removing the need for the cap at the overflow bottle. The only issue then would be to replace the dummy cap at the top of the thermostat housing with the bracket in your picture (with overflow line nipple).

 

yes

 

Big thanks to Rotary Resurrection for teaching me that you can just switch the cap housing from the overflow bottle to the thermostat housing in order to delete the second cap. Attached is the process / finished product.