This is a good solution: using two envelopes for one letter. The sender first mails it to the re-publisher, who opens the outer envelope and then sends out the inner envelope. The re-publisher can be established as a standalone service. 
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If the re-publisher uses the same receiving address, it seems that two envelopes are not necessary. Instead, a single envelope can include two receiving addresses (a feature supported by Nostr). The first receiving address is the re-publisher's receiving address, and the second address is the true recipient's nearly one-time receiving address. The re-publisher receives the message from relay X and then forwards it to relay Y. The recipient retrieves the message from relay Y.