Yes, you're correct... the S2 Stagea ECU SHOULD fit the R34, but you'll loose traction control and probably Air Con... but it may be a different plug, so maybe not... Same reason the S2 ECU doesn't fit in the S1 Stagea
If you're looking for an ecu, i would suggest you join the Facebook group "RB 20/25/26 South Easy Qld Parts for sale"... there are always R34 ECU's on there... just post a WTB. I would have had one spare i could have sold you, but a guy didn't come through, so i only have one on me at the moment. I'm trying to buy them up when ever i seem them for sale