Ive not every tried this specifically in this case, so this is a theory response. However I think you "should" be able to buy the domain and pass the link to you. If ranks go up, leave it, if they go down, remove it.
Im just a big believer in letting things be what they are, aka push the button and see what happens.
The upside to that is that I learn a load of things in a practical way, including short cuts. Im not afraid to push the button and see what happens.
The down side to that is... Well on a grand occasion when you push the button, something blows up.
So you can do as you please and its worth some input from someone who has tried it in this case. because I suppose in theory there is a chance if you buy it, place the link and ranks go down, and you remove the link, that google could still keep ranks down, its certainly possible, so that would be my only concern . So someone who has done that could shed some insight.
Of course I always test everything, so personally I would buy it because its just an investment of your time and like $10 for the domain. So if you have the time and the $10, buy it. Point it at some other site, see what happens. If its positive, remove that link and point it at yours, if its negative, remove that link and then just let it lay.
My 2 cents.
