I know it's an old topic but I did an ej22 swap in a brat a few years ago and never did get the original oil pressure gauge to work. I've heard the ea81 and ea82 senders are the same. How did the oil filter sandwich work out for you? Do you have any pics with the oil pressure sender installed?
This is how I've done mine - stock EA oil warning light sender on one end and aftermarket mechanical gauge on the other. It all fits under my alternator. To fit your pressure sender unit which is much larger, I've seen it done with a line extension to an area where it's easy to access and clear of any other objects. Worked a treat for them.