Well, no miracle happened so far. My original head still can't print PGBK, and the 'new' one's PGBK nozzles are half-clogged despite extensive soaking.
I have now acquired a second 'new' head from a similar source and am soaking it as we speak (the definition of insanity, they say). Speaking of which (the soaking, I mean), it seems to me that, however much I soak these buggers, there's always some ink left (as evidenced by shaking the head. after which some drops invariably appear on the top filters). Perhaps it is simply impossible to completely clean a head via soaking and flushing?
If so, won't trying to dry the head afterwards mean that whatever ink is left dries up clogging the nozzles?