Once someone has pledged an object as hekdesh, what is his relationship with that object? He is able to redeem it, so does that mean that he still has some kind of connection to that object? If not, can anybody redeem an object that someone else has made hekdesh?
It seems from the Mishna in Arachin (25a) and the Rambam (Hilchot Arachin 4:19,20) that anyone has the ability to redeem a field that has been pledged as hekdesh. The only difference between the original owner and a secondary individual redeeming the field would be the need to add a fifth to the redemption price and the fact that if the original owner redeems the field, it would be his even after the yovel (ie. return to his ancestral estate) and if a secondary individual redeems the field it would revert to hekdesh after yovel (ie. not become a part of his or the original owner's ancestral estate).
Another interesting question, which is related to yours: If the original owner wants to redeem the field against the will of the temple treasurer would he be able to do so? If the answer is yes (and a secondary party is not able to do so) I would have to say that there are still some "rights" of the original owner to the field because of the fact that we try to avoid loss to ancestral estates and therefore allow a person to redeem his field even against the will of the temple treasurer in order to preserve his ancestral estate.