Is that a standard Subaru oil gallery plug in the pic, if so it would be a good place to tap for oil pressure. It is what most Subaru guys do on th EJ engines since there is an extra unused plug. You just buy the oil pressures switch plug which has a 1/8 PT hole in it to replace the solid plug.
Again I would not use a Tee screwed into the front ports, those things have a tendency to crack under repeated stress without much support.
You can even buy this one which is threaded for NPT and not BSPT. Depends on the gauges and send unit you are using.
http://www.fastwrx.com/oilgalleryplug.html
Or just get the whole relocation kit since the pressure senders are usually pretty large.
http://www.fastwrx.com/reoilprsekit.html
This Tee I would see working but it's expensive
http://www.nissport.com/catalog/prod...e=Applications