Any encoding method requires a fixed (usually hard-coded) set of "digits" and if you don't know those you won't be able to decode it. For example if you don't know base16 uses 0-9, a-f you con't decode it. For base2048 you need the digits which in case of a BIP39 mnemonic or what I explained above is the word list that would be required to decode both the mnemonic and the encrypted mnemonic alike.

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i'd much prefer a solution that
1)didn't require a fixed wordlist
2)allowed for variable length mnemonics
3) but still operated by changing words in to other words
4) encodes the derivation path being used so that someone can find their money
5) encodes the particular cryptocurrency that the derivation path above refers to
6) possibly allows for encoding multiple 4/5 pairs of data so that different coins can be referenced
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1-3. You still need to know the word list but it could be changed in a way that doesn't depend on a fixed length word list like the algorithm Electrum uses
4-6. This can also be implemented but it would increase the number of words and the result will no longer resemble a normal BIP39 mnemonic.