Pages 49 and 50 of this document (2005 edition, no doubt it's been updated, but)...
http://www.gofir.com/general/north_a...MNPSA_2005.pdf
There's no mention that SLOP should only be used on an organised track, merely that it's allowable in Oceanic Airspace:
"... Consequently, it has been determined that allowing aircraft conducting oceanic flight to fly lateral offsets will provide an additional safety margin.....".
and, as the north Atlantic regulars will know,
:
"There is no ATC clearance required for this procedure and it is not necessary that ATC be advised."