Yes, you can cover jam with greaseproof paper.
When I do so, I cut a smaller circle of paper and lay it directly on the surface of the jam.
It is a good thing to dampen the paper with the preservative you use in the jam.
If you want a nearly airtight covering, you can put a layer of cling film over the outer layer of greaseproof paper.
Have any of you got all electric cars? Pros and cons please.