RE:Bidirectional ESD SCR
Please see at Sofics solutions for HV (e.g. WP-2010-Q1-1.pdf). Such unsymmetric ESD clamps are oftenly used for HV design. E.g. TI used such protection for some old 0.35um families (see scla013d app.note e.g.). Ideology of such ESD protection can be defined as "Common Discharge Line",- when each I/O pin have identical ESD clamp (based on SCR, NPN, Zener, ...) to common line (GND typically). Distributed active power clamps can improve protection significantly for long distant pins.