Liki has the right idea here; I find it very unlikely that you will be able to do this. In theory it's not a bad idea (a simple loan with interest), but in practice I think it would be too hard to pull off. There is too much risk on both sides (for the person giving you the currency, and for you if you offer collateral, as there's no guarantee you'd get it back). You may know that you are a trustworthy person, but others won't have any way of proving that. Additionally, I see most people who have that much currency using it themselves, unless they already have all the relics they need. Sure, they'd get it back and then some, but for me it would be very hard on holding off on getting a relic for an indeterminate amount of time, even if I were confident that I would get paid back. I guess it doesn't hurt to try, and I truly hope you can pull it off (it is always nice for me to see others being honest and honorable online), but I don't want you to get your hopes up too much. You will likely end up having to sponsor runs and/or buy currency. Your way is certainly faster, but just like in real life, people have to weigh the risks against the potential gain. I wish I could help you out myself, but I cannot. Best of luck to you though.
To offer some more constructive advice, you might try aiming for a more manageable amount of currency, e.g. 10 one hundred pieces with payback of 12 or something. This reduces the risk considerably (though it is still substantial). If you can get someone to do that and somehow prove to them that you are trustworthy, you may be able to convince them to go the rest of the way. Also, providing a concrete timeline for repayment might help as well.
Edited, Apr 5th 2011 6:03pm by Daimakaicho
Formerly of Lakshmi, now in Asura
FFXIV: Kagu Tsuchi