Can the ECU's boost control solenoid (BCS) be used as a boost controller?

No. Or, at least, not directly. The operation of the BCS has virtually nothing to do with intake pressure. The only method of doing this is to disconnect the BCS and wire it to a customized boost controller. Theoretically, it could be done by by reprogramming the ECU, but the new ECU would also have to accept inputs from an additional intake pressure sensor. To date, nobody has created such an ECU modification.