Rougue Leader, you are right. I totally missed the element that the damper will remain fully extended. Glad you pointed it out before I tried it and had one of those 'Duh' realisations. Would be confident the compressors wouldn't slip off as they would hook around the curved bottom of the strut and the spring coils but the exercise itself would be totally pointless, as you have correctly pointed out - unless something like a bottle jack was used to squash the damper too - wouldn't lift the car with the spring compressed.
Personally not too keen on the plank idea stability wise so have got round it here by buying a pair of wide car ramps to safely raise the rear wheels enough to gain bolt access, while the front can be raised with a trolly jack and axle stands. Problem solved, at last.