I just wrote a review of the new DVD documentary Sounds And Silence. It's a good movie, following Manfred Eicher to various gigs and recordings. It starts with footage of Eicher recording an Arvo Part session in a church in Finland. You should watch it.
To be honest, it doesn't look like he's doing anything particularly special: just your standard mic'ing that is routinely discussed in the Remote Possibilities thread. If anything, he probably uses a lot more
mics than the classical/jazz location purists would.
It looks like 1 main stereo spaced pair (couldn't tell, but would imagine Schoeps or DPAs) and then various spot mics. I spotted a few C414s in there.
In other footage you see Eicher recording various other ECM artists in different studios across the world. Eicher comes across as very hands on, in terms of editing and mix and production, but I was surprised to see there didn't appear to be any "big secret" type recording tricks. I suspect the Arvo Part recordings have a lot to do with the space they're recorded in.
If I were you I would buy two omni condenser mics. I know you say you can't afford it, but could you not afford 2 of these, say: Red12 - 12 Gauge Microphones
(Where do you live? Can you not find a church hall with a piano or something? Or a conservatoire rehearsal space you could use?)
If not, and you are really stuck with your room and two SM57s, there's not a lot you can do other than just move those two mics until they sound good. But make sure your piano lid is as open as possible.
PM me your address if you'd like the DVD: I doubt I'll watch it again. I was going to give it to the charity shop.