Good questions! I like the "event seed" option you mention.

Without relay hints, it's just a game of brute force searching relays. But perhaps we could encode the relay somehow. TBH that's a bit trickier. The primary ID of a relay is its URL. Encoding all of it would take too many words. But expressing the relay URL in plaintext would betray the purpose of the preceding 6 words, which would reduce this scheme's effectiveness against shadowbanning. Perhaps it's up to the user. Include the relay if you aren't in an adversarial environment, and exclude it if you are; without a relay hint you'll simply have to brute force search relays for it until you find it, which can be somewhat automated. If you are sharing an event in such a furtive way, you might want to broadcast your event to as many relays as you can in order to ease the pain of finding it.