Ah-ha!
Just in case anyone's interested a kind person's pointed me in the direction of some A5000 schematics & the v. good news is none of the above faffing about will be required.
There's two wholly separate tracks leaving the clock pin from the offending crystal going off to memory & video controllers (via a fair bit of logic, but who cares about that?).
As the two lines separate @ source not within some logic somewhere I can just cut a track & plumb in another crystal - job done.
