The reason for key strengthening is that passwords don't have as much entropy as is expected for the key. The time it takes to break a key is proportional to the number of possible keys. Strengthening algorithms used on passwords compensate for the poor entropy by increasing the proportionality constant. But the gap is so large that strengthening cannot compensate. A password with 64 bits of entropy is very hard to memorize — that's 10 random printable ASCII characters (if you start imposing memorable features, the entropy goes way down) or 14 random lowercase letters, but to reach 128 bits of “cracking difficulty”, which corresponds to the most common key size for symmetric encryption, you'd need a whooping 2^64 — about a century of CPU time.
In your scenario, a memorable password is the wrong method. It would be a password that you rarely use, yet needs to be very strong and thus hard to memorize.
Use a random key, instead of a password-derived key, and print it out. Store the printout in a secure place.
If the software you use requires a password, generate a random password with as much entropy as the key. 20 ASCII printable characters, or 28 random lowercase letters, or something equivalent, give you 128 bits of entropy. If technology progresses to the point that this is breakable, the key you're encrypting will probably not be secure anymore anyway. Since your password has enough entropy for a key, you don't need to strengthen it. Once again, print it out.
Once you go that route, you might as well print out the original key and forget about uploading it. But adding a level of indirection does have advantages. It means that an adversary needs to obtain both a copy of the encrypted key and a copy of the key encryption key. With this separation, if one of the parts is compromised, you may have time to act before the second one is also compromised. You can also store the key encryption key in several separate pieces. There are secret sharing algorithms that allow splitting a key in n pieces such that it can be recovered from any k pieces, but no fewer. If you want to keep things simple, you can generate multiple key encryption keys, upload the master key encrypted with each of these, and split the key encryption keys and dispatch the pieces so that it takes more than a single place to recover the whole thing.