Well, I don't think you have to keep it online all the time. I'm sure you can use a hardware wallet for that, @furion wrote an article about that: https://steemit.com/trezor/@furion/build-yourself-a-trezor
Also @jesta has a local wallet (an app you download) called Vessel: https://github.com/aaroncox/vessel/releases
Thanks for pointing that out to me. I'll have to read those posts a few dozen times to understand them. I have access to a couple of T61 laptops, so may give it a try.
STEEM converted to STEEM POWER obviously has to be on the platform to count toward your voting strength. Being able to keep 100k SP offline and still count would be good, not that I'm anywjete near that, yet...