Yes, that's a bit of a problem on the average seedbox. You'd have to modify your torrent to seed on I2P by adding I2P trackers (just a couple of them, nothing much), and then run either BiglyBT or I2PSnark to seed them on I2P. Unfortunately, most seedboxes don't give you root access, neither do they bundle these apps. Qbittorrent doesn't have good support for it yet unfortunately.
If you have an SBC/spare computer at home, would be great if you could attach a hard drive to it, install i2p/i2pd and either of the mentioned torrent clients, and seed from there in the meantime. Qbittorrent has seen community interest in I2P, unfortunately it's just not there yet
I do seed stuff (I downloaded a TV show of around 40GB last month and I've been seeding it since). It just keeps seeding at full speed till the time they throttle my bandwidth. Last I checked I was over 7 in ratio for that thing, but whatever. It works out because I don't leech much
I will eventually move to a VPS provider who doesn't mind public torrents (I'll pay through XMR). This seems like a much better idea since my needs have diversified and I'd like everything together on one machine to save costs. There's other ways to use your seedbox too (without root access) and seed stuff that people really need/are deprived of (vague description on purpose).