In reply to deanr:
Really not a suitable tool for the job. I'd buy the cheapest short single rope I could find.
Try gradually, progressively, testing out how far off the ground you have to be before you don't deck toproping on one half rope. I'd guess somewhere around 3 to 4 metres. Maybe less if your belayer keeps the rope so bowstring-tight that you're not actually climbing the route anyway.
Also, abrasion at the top would be a *huge* concern with a soft thin stretchy rope.