This can not work. Either it will destroy all casual players (because people who log in less than every day will end up losing the majority of their inventory), or you will just introduce a new irritation factor. Every day, everyone has to log in all their alt banks to refresh their timers. There is no practical way of intuiting mains from alts other than login data. Add a "portage" skill, than people will just have to "build" alt banks. Humors? Same issue, people will just build Alt banks. Most login statistics can be faked as well, and the more requirements you put, the more you penalize casual players who are less likely to qualify with their actual mains.
Alternatively, If your intention is to force everyone to empty their inventory before they log out on all their characters (main or alts), then I wouldn't bother with any sort of elaborate decaying system, just have every character drop their inventory on log out (onto the ground, like they were KO'd). That would be a much better system than the one you describe and would eliminate alt-banking entirely. I'm not wildly fond of the idea, but that might be inertia on my part. Intellectually it seems sound. Realistically, you'd want to put in a delay, 15m, I'd say, before the items dropped in case someone crashed rather than logged out. (I recently had my computer crash hard-core playing Dota, and I didn't get rebooted in time to rejoin before the 5m mark, so I'm sensitive to reboot time issues

. The real problem here, of course, is that you are really changing the fundamental nature of the game. Without alt-banking, players would have to come up with new schemes to offset potential setbacks. I can think of a few already... Not sure it wouldn't make the game better, but it would likely discourage players even more and put an even greater strain on the already weak static defense mechanics. One must assume this solution occurred long ago to Seatribe and was discarded, so I really doubt you will see any movement on this issue.